Can, could and be able to: ability, possibility and permission

Start with the form

Can and could are modal verbs. They come before another verb and help give its meaning. Use the base form, such as read, go or be. Can and could stay the same with I, you, he, she, it, we and they.

Do not add -s or put to directly after can or could. Do not add do, does or did to make a question or negative.

• My sister can repair clocks.
• Could your sister repair this clock?

Write the ordinary full negative of can as one word: cannot. Can't is its short form. Could not is two words; couldn't is its short form. Full negatives can give emphasis, and Cambridge also describes them in formal contexts.

Short answers

In an ability question, use the same modal in the short answer. You do not need to repeat the main verb.

A request or permission question needs a reply that fits its purpose, not just a repeated modal. You will see that difference below.

What you can do now

Use can for a skill you have learned or something you are able to do. Can't says you do not have that ability.

• I can sew a button on a shirt.
• My cousin can't ride a bicycle yet.

Can also describes what is possible in the situation, not only a learned skill.

• We can meet at lunch because we are both free.
• You can charge your phone beside the desk.

The first describes an available time. The second describes a facility you can use. Neither says that the action has already happened.

Seeing, hearing and understanding

You can use can with see, hear, smell, taste and feel, and with verbs such as understand and remember. Here it describes what you experience or understand now.

• I can hear a dog outside.
• Can you understand the last instruction?

These are not claims about a skill you have practised. Read the verb and the situation together.

Requests, offers and permission

Asking someone to act

Can you plus a base verb can ask someone to do something. Could you is another request form. British Council and Cambridge describe could as more polite than can in this use. Could does not put the request in past time.

• Can you move your bag, please?
• Could you email the receipt today, please?

You can accept with Of course or Yes, I'll do that. If you cannot help, explain: Sorry, I don't have the receipt. A request asks for action, not only a report of someone's ability.

Offering help

Can I or Can we can offer to do something. I can and I could can also offer help when the situation makes that meaning clear.

• Can I carry the smaller case for you?
• I could collect your parcel if that helps.

The first asks whether you want the help. The second puts forward an offer; it does not report a past collection.

Asking for and giving permission

Can I asks whether you are allowed to do something. Could I is a more polite or formal alternative.

• Can I sit beside the door?
• Could I use the meeting room for an hour?

To give permission now, use can or an answer such as Yes, of course. Do not mechanically repeat could as the ordinary permission reply.

You could leave it here can suggest an option, rather than directly give permission. Could also appears when you report earlier permission: The receptionist said we could leave our coats there. These meanings are different, so could is not wrong in every sentence about permission.

Can't can mean that a rule forbids something, not that a person lacks the physical ability.

• You can't take drinks into this room. a rule

See Requests, offers, permission and invitations for more request, offer and permission patterns.

Past ability

Use could for a general ability in a past period. Couldn't says that you did not have an ability or could not do something then.

• At ten, I could name every planet.
• Before the lessons, I couldn't read music.

Could does not by itself say that the ability has disappeared. As a teenager, I could swim long distances tells you about that past period; you need more information to know what the person can do now.

These are source-specific records, not a ban on meeting could earlier.

One completed success

When you tell someone that you succeeded on one particular past occasion, normally use was able to, were able to or managed to. Managed to draws attention to succeeding at something difficult.

• After three attempts, I was able to open the cabinet.
• We managed to move the sofa through the doorway.

These sentences report completed successes. In contrast, At that time, I could open the cabinet describes an ability or opportunity; it does not itself report that particular successful attempt.

Negatives and exceptions

For an unsuccessful occasion, couldn't, wasn't able to, weren't able to and didn't manage to can all be possible. Choose the one that fits the sentence and meaning.

• I couldn't reach the shelf, so I asked for help.
• I wasn't able to reach the shelf, so I asked for help.

Do not turn the completed-success guideline into a ban on could in every sentence about one occasion. With senses or mental processes, could can describe a particular past experience.

• When the music stopped, I could hear the clock ticking.
• After her explanation, I could understand the drawing.

British Council also confirms the pattern in I'm glad you could come: it can refer to someone arriving for one event. The clause pattern and meaning matter, not just whether the event happened once.

Be able to: change be, keep able to

Be able to expresses ability. It is useful where can has no grammatical form, but it is not a replacement for every meaning of can. In You can't park here, the meaning may be a rule, not inability.

Unlike can, be changes with the subject and time. Make its questions by moving be before the subject. Add not after be for negatives.

Cambridge describes be able to as a more formal alternative to can in present ability uses. It is not necessary to replace every can sentence with it.

Future meaning is not the same as a future form

Can can refer to the future: I can help you tomorrow is grammatical. Do not write that can is impossible with tomorrow or next week.

Can has no will form. If you need will, use will be able to, not will can. This can describe a future ability or opportunity.

• After the repairs, we will be able to use the lift again.
• Will you be able to travel by then?

Can for a future arrangement or available action is also supported by Cambridge and British Council.

Perfect forms and other verbs: later extension

Can has no past participle to put after have. For a period up to now, use have or has been able to. The negative is haven't or hasn't been able to.

• I haven't been able to contact the owner this week.

This covers the period up to now. I couldn't contact the owner yesterday places the problem in a finished past time.

After to or another modal such as might or should, use be able to. Do not put two modals such as might can together.

• She hopes to be able to return in June.
• We might be able to borrow a larger car.
• You should be able to find the number on the letter.

Might be able to describes a possible ability or opportunity. It does not promise that you will borrow the car.

Possibility, suggestions and imagined situations

General possibility

Can can say that something happens sometimes or is possible in general. It does not say that it always happens.

• This path can be muddy after rain.

This describes a tendency. This path could be muddy today presents a possibility about this occasion. Could does not itself make that sentence past.

Do not use affirmative can as a simple replacement for maybe in a guess such as Maybe Ana is at home. Use Ana could be at home. But can is valid for an available action: Ana can work at home tomorrow. The meaning, not just the future date, decides.

Suggestions

Could can suggest an option. Can can also suggest an available option. Do not label one wrong merely because the other is possible.

• We could take a picnic instead of buying lunch.
• We can take a picnic; there is space in my bag.

Could can describe what would be possible in an imagined situation, rather than a past ability.

• If we had a second car, we could travel separately.

Here could means would be able to or would have the option to. It does not say that you have a second car. See The second conditional: unreal situations now or in the future and May and might: possibility and other meanings.

Other meanings and forms: later extensions

Reporting can

In reported speech, can can become could. Keep statement order after said that or asked whether.

This is reporting, not necessarily an ability that has ended. A change is not compulsory in every still-relevant report. See Reported statements: tense changes and say or tell and Reported questions and requests.

Impossibility and deductions

Can't can express a belief that something is impossible, not a rule or a lack of skill.

• That can't be our taxi; ours is blue.

The colour is evidence for the speaker's conclusion. Keep those source bands separate. See Must and can't for deductions for the full contrast.

Could have and passive forms

Could have plus a past participle looks back at a possibility or opportunity. It can also make a guess about the past. The surrounding words decide whether the action happened, did not happen, or is unknown.

• We could have booked the hall, but we chose the garden.
• The keys could have fallen behind the cupboard. a guess, not a known fact

See Could and could have: possibilities and missed chances for the full reference.

A passive can put the thing affected first: The screen can be cleaned with a soft cloth. The pattern is can plus be plus a past participle. See The passive in many tenses and with modal verbs.

What comes next

Use Requests, offers, permission and invitations for requests and permission, May and might: possibility and other meanings for uncertain possibilities, Could and could have: possibilities and missed chances for past possibilities and opportunities, and Must and can't for deductions for conclusions from evidence. Continue with The second conditional: unreal situations now or in the future for imagined situations and Reported statements: tense changes and say or tell for reporting.