Modal Verbs

Lesson 1: Can and Could

Lesson: 1 of 9 | Level: 🟠 Intermediate

1. Lesson Overview

Can and could are the most frequently used modal verbs in English. At first glance, they seem straightforward — can for present ability, could for past ability — but their actual range of meanings is considerably richer than this simple description suggests. Both verbs express ability, possibility, permission, and a range of other meanings that vary significantly depending on context, register, and the form of the verb that follows them.

The relationship between can and could is not simply one of present vs. past. Could is used for past ability, but it is also used for present possibility, polite requests, hypothetical situations, and suggestions — contexts in which it has nothing to do with the past at all. Understanding the full range of meanings of both verbs, and the subtle distinctions between them, is essential for accurate and sophisticated use of English at intermediate and advanced level.

Objectives

By the end of this lesson, you will be able to:

• Identify and apply all the main meanings of can and could
• Distinguish between the uses of can and could in present and past contexts
• Understand the register and politeness distinctions between can and could
• Recognise and correct common errors in the use of can and could

2. Core Content

A. Forming Can and Could

Can and could are modal auxiliary verbs. They are followed by the bare infinitive and have the same form for all persons.

Affirmative, negative, and question forms

Note that cannot is written as one word — not can not — in standard British English, except when not is being stressed for emphasis: I can not go — I simply refuse to.

B. The Main Uses of Can

1. Present ability

Can is used to express ability — what someone or something is capable of doing at the present time.

For example:

The latest generation of autonomous underwater vehicles can operate at depths of more than 6,000 metres without a crew. Some species of tardigrade can survive exposure to the vacuum of outer space, extreme radiation, and temperatures close to absolute zero.

2. General possibility

Can is used to express general possibility — something that is theoretically or generally possible, even if it does not always happen.

For example:

Volcanic eruptions can occur without warning along any section of the Pacific Ring of Fire. Prolonged exposure to deep-sea pressure can cause a range of physiological effects in human divers.

3. Permission — informal

Can is used in informal and everyday contexts to give or ask for permission. It is less formal than may but is widely accepted in all but the most formal situations.

For example:

You can access the archived data from the server once you have received clearance from the data management team. Can I use the laboratory equipment for the additional experiments I am planning?

4. Requests — informal

Can is used in questions to make requests — asking someone to do something. It is less formal than could and is most appropriate in informal and everyday contexts.

For example:

Can you check the calibration of the instruments before the dive begins? Can the committee provide a response to the proposal before the end of the week?

5. Perception and sensation — with stative verbs

Can is frequently used with stative verbs of perception — see, hear, smell, taste, feel — to describe what the speaker is currently perceiving. This construction avoids the present continuous with stative verbs.

For example:

I can see the outline of the hydrothermal vent through the submersible’s cameras. The researchers can smell sulphur as the submersible approaches the vent field.

6. Negative deduction — logical impossibility

Cannot (can’t) is used to express a logical deduction — the speaker’s confident conclusion that something is impossible or untrue based on available evidence.

For example:

The temperature readings can’t be correct — no organism known to science can survive at 400°C. These fossils cannot be from the Jurassic period — the geological strata they were found in are Cretaceous.

7. Characteristic behaviour and general truth

Can is used to describe characteristic or typical behaviour — what something is capable of or known to do under certain conditions.

For example:

A blue whale can consume up to four tonnes of krill in a single day during the feeding season. Deep-ocean currents can travel thousands of kilometres before returning to the surface through the process of upwelling.

8. Offers and suggestions

Can is used to make offers and suggestions — indicating willingness or readiness to help.

For example:

I can help you with the data analysis if you need additional support before the deadline. The institute can provide logistical support for the expedition if funding is secured.

C. The Main Uses of Could

1. Past ability

Could is used to express ability in the past — what someone or something was capable of doing at a past time. This is its most basic meaning.

For example:

Early submersibles could reach depths of only a few hundred metres — modern vehicles can descend to more than 10,000 metres. Darwin could identify hundreds of species of plant and animal from direct observation, without the benefit of modern classification tools.

Note an important distinction: could expresses general past ability — the capacity to do something. For a specific completed action on a single occasion in the past, was able to or managed to is used instead of could.

The team was able to collect samples from the deepest site despite the difficult conditions. (specific completed success — not could collect) The submersible could reach 6,000 metres in the 1990s. (general past ability — could is correct)

2. Possibility in the present or future — tentative

Could is used to express present or future possibility — something that is possible but uncertain, or that the speaker considers less certain than can would imply.

For example:

The anomalous readings could indicate the presence of a previously unmapped hydrothermal vent field. The expedition could encounter severe weather conditions during the crossing of the Drake Passage.

3. Polite requests

Could is used to make polite requests — more formal and more tentative than can, and therefore more appropriate in formal, professional, and academic contexts.

For example:

Could you review the draft methodology section before it is submitted to the committee? Could the institute provide a detailed breakdown of the projected costs for the second phase of the expedition?

4. Polite permission — asking

Could is used to ask for permission in a polite and indirect way — more tentative and deferential than can.

For example:

Could I use the electron microscope for the additional analysis I am planning? Could we schedule a meeting to discuss the preliminary findings before they are presented to the board?

5. Suggestions

Could is used to make suggestions — offering a possibility for consideration without pressing for a commitment.

For example:

The team could conduct an initial survey of the southern sector before committing to the full mapping programme. Scientists could use satellite data to identify potential vent locations before deploying submersibles.

6. Hypothetical and conditional situations

Could is used in hypothetical and conditional sentences — expressing what would be possible if certain conditions were met.

For example:

If the team had access to more advanced equipment, it could conduct a far more comprehensive survey of the vent field. With additional funding, the research programme could extend its monitoring network to cover the entire Indian Ocean basin.

7. Criticism and reproach — could have

Could + have + past participle expresses criticism or reproach about a past action — implying that something was possible but was not done. This is examined in detail in Lesson 9.

For example:

The team could have collected additional samples if it had allocated more time to the southern sector. Scientists could have established baseline measurements much earlier if monitoring had begun in the 1970s.

8. Negative deduction — impossibility in the past

Couldn’t + have + past participle expresses a confident deduction that something was impossible in the past.

For example:

The species couldn’t have evolved in isolation — the genetic evidence shows clear connections to populations on adjacent islands. The readings couldn’t have been accurate — the instrument had not been calibrated for three months.

D. Can vs. Could — Key Distinctions

3. Usage in Context

• Can expresses present ability — what someone or something is capable of doing now.

The latest generation of autonomous underwater vehicles can map the ocean floor with extraordinary precision, covering hundreds of square kilometres in a single deployment. Some species of deep-sea bacteria can survive in conditions of extreme heat, pressure, and acidity that would be instantly lethal to virtually every other known form of life.

• Can expresses general possibility — something that is theoretically or characteristically possible, not necessarily happening now.

Earthquakes can occur without warning along any section of a major tectonic fault line. A single volcanic eruption can inject enough sulphur dioxide into the stratosphere to lower global temperatures by half a degree for several years.

• Can is used in informal contexts to give or ask for permission — less formal than may but widely accepted in everyday English.

You can access the full data set once you have received written authorisation from the principal investigator. Can I submit the analysis in two separate documents rather than as a single combined report?

• Can is used in informal requests — asking someone to do something in a direct but not impolite way.

Can you send me the latest version of the data before the meeting this afternoon? Can the technical team check the submersible’s power systems before the next dive?

• Can is used with stative verbs of perception — see, hear, smell, taste, feel — to describe current sensory experience without using the present continuous.

I can see the outline of the vent structure clearly through the submersible’s forward camera. The divers can feel the increase in water temperature as they approach the hydrothermal vent field.

• Cannot (can’t) expresses logical impossibility — the speaker’s confident deduction that something is impossible or untrue given the available evidence.

The temperature reading can’t be accurate — no known material could remain intact at that temperature for more than a fraction of a second. This species can’t have evolved independently on both islands — the genetic similarity is too great for parallel evolution to explain.

• Can describes characteristic or typical behaviour — what something is known or expected to be capable of under certain conditions.

A blue whale can consume several tonnes of krill in a single day when feeding conditions in polar waters are optimal. The deep Atlantic current can travel from the surface of the North Atlantic to the floor of the South Atlantic and back again over a period of approximately one thousand years.

• Could expresses general past ability — what someone or something was capable of in the past. For a specific completed success, was able to or managed to is used instead.

Early submersibles could reach depths of only a few hundred metres — a fraction of what modern vehicles are capable of. Despite the difficult conditions, the team was able to collect all the samples from the deepest site before the weather forced them to surface. (specific completed success — not could collect)

• Could expresses present or future possibility in a tentative, uncertain way — less assertive than can.

The anomalous heat signature detected by the sonar could indicate the presence of a previously unmapped submarine volcano. The data inconsistencies could suggest that the monitoring instruments were affected by the seismic event recorded last month.

• Could is used for polite requests — more formal and indirect than can, and therefore more appropriate in professional and academic contexts.

Could you review the draft report and provide your comments before the end of the week? Could the committee clarify its position on the funding criteria before the application deadline?

• Could is used to ask for permission politely — more deferential and indirect than can.

Could I have access to the archived survey data for the purposes of the comparative analysis? Could we arrange a meeting with the principal investigator to discuss the preliminary findings before they are submitted for publication?

• Could makes suggestions — offering a possibility for consideration in a tentative and non-prescriptive way.

The team could conduct a preliminary survey of the northern sector before committing resources to the full mapping programme. Scientists could use existing satellite thermal imaging data to identify the most promising sites for submersible deployment.

• Could expresses hypothetical and conditional possibility — what would be possible if certain conditions were met.

If the research programme had access to a larger fleet of autonomous vehicles, it could survey the entire vent field in a fraction of the time currently required. With a more generous funding allocation, the team could extend the monitoring network to include sites in the southern Indian Ocean.

• In formal and academic writing, can is used to express what is theoretically or empirically possible — what the data, the evidence, or the methodology permits.

The model can predict temperature changes with an accuracy of approximately ±0.1°C under controlled conditions. The technique can detect concentrations of dissolved methane as low as one part per billion in deep-ocean water samples.

• In formal writing, cannot is preferred over can’t — the contraction is appropriate in informal and neutral contexts but should be avoided in academic prose.

The current monitoring network cannot provide the spatial resolution needed to detect localised changes in vent activity. (formal) The current network can’t detect those changes. (informal)

• Could in negative form — couldn’t — expresses past inability, past logical impossibility, or a polite and indirect refusal.

The early monitoring instruments couldn’t detect changes in ocean acidity below a certain threshold — modern sensors are far more sensitive. I’m afraid I couldn’t attend the briefing — I was conducting fieldwork at the remote monitoring station throughout that week.

• Do not use can to describe a specific completed achievement on a single past occasion — use was able to or managed to instead.

Despite the severe weather, the team was able to complete the survey of the southern sector before returning to port. (not could complete) The submersible managed to reach the target depth on the third attempt after two equipment failures. (not could reach)

• Can and could are both used in reported speech — can backshifts to could when the reporting verb is in the past.

The scientist said that the new submersible could reach depths that had previously been considered inaccessible. The team leader confirmed that the vehicles could operate continuously for up to seventy-two hours on a single charge.

• In the passive voice, can and could are followed by be + past participle.

Deep-sea sediment cores can be analysed to reconstruct climate conditions going back hundreds of thousands of years. The vent ecosystem could be permanently damaged by large-scale mineral extraction before its full ecological significance has been determined.

4. Common Errors and Corrections

5. Lesson Mastery

After completing this lesson, you should now be able to:

    ✅ Identify and apply all the main meanings of can and could

    ✅ Distinguish between the uses of can and could in present and past contexts

    ✅ Understand the register and politeness distinctions between can and could

    ✅ Recognise and correct common errors in the use of can and could