Can, Could and Be Able To: Differences, Uses and Exercises

Learn how to use can, could and be able to in English: differences, common mistakes, Cambridge exam tips and exercises with answers.

Ability sounds like a simple idea until you have to put it into English. Suddenly there are three ways of saying it, can, could and be able to, and choosing the wrong one is one of the mistakes I correct most often in my students’ writing and speaking.

The good news is that there are very few rules and they are clear. In this guide you will see what each form means, when to use it, the difference between could and was able to (the one that separates a B1 from a B2), the mistakes learners keep making, how all of this turns up in the Cambridge exams and, at the end, forty practice sentences with answers. Let’s get started! 🙂

What modal verbs are (and the three rules you must not forget)

Can and could are modal verbs, a special family that also includes must, should, may, might and will. They do not behave like ordinary verbs, and this is what to fix first, because almost every mistake comes from here.

• They take no -s in the third person. It is she can swim, never «she cans swim».
• They need no do or does for negatives and questions. The modal does the job itself: Can you swim?, I can’t swim. If you want to revise how questions work, it is all in my guide to Yes/No and Wh- questions.
• They are followed by the infinitive without to. I can help you, not «I can to help you».

And there is a fourth, less famous rule, the one that explains why be able to exists at all: two modals cannot sit together. That is why «I will can» is impossible in English, and so are «to can» and «canning». When the grammar asks for an infinitive, a participle or a gerund, can runs out of forms and needs a hand. That hand is be able to.

CAN: how to use it

Can is the present form. It is extremely common and it has four main uses.

1. Ability in the present

The star use: what you know how to do.

• I can ride a bike, but I can’t drive.
• My sister can speak three languages.
• Can you cook?

2. Permission

For asking for and giving permission in ordinary, everyday register.

• Can I sit here?
• You can use my laptop if you need it.
• You can’t park here. (prohibition)

3. General possibility

When you talk about what happens sometimes, about what is possible in general.

• Granada can get really cold in January.
• Exams can be stressful.

Be careful with this one, because here can does not mean «know how to», it means «this happens sometimes». If you want to speculate about one particular situation, English uses other modals (may, might, could), never can: you say He might be ill, not «He can be ill».

4. Requests and offers

• Can you help me with this, please?
• Can I give you a hand?

Negatives and questions

The negative is cannot, which in practice almost always appears contracted as can’t. Notice that cannot is written as one single word: it is one of the very few negatives in English that work that way.

COULD: how to use it

Technically could is the past of can, but calling it only that sells it short: it also works in the present and the future. These are its uses.

1. General ability in the past

What you knew how to do, or what you could normally do, over a period of your life.

• When I was five, I could already read.
• My grandfather could drive until he was 85.
• I couldn’t swim until I was ten.

Pay attention to the word general, because it is the key to the most important section of this post. We are not talking about one particular occasion here, but about an ability that lasted over time.

2. Requests and permission, more politely

Could sounds softer and more formal than can. It is the one you want with a stranger, in a shop, or in the part of the speaking test where you talk to the examiner.

• Could you repeat that, please?
• Could I ask you a question?

Think of it as a politeness ladder: Can you…? is fine among friends, Could you…? is more polite, and Would you mind…? is the most formal of the three.

3. Possibility or suggestion in the present and future

Here there is nothing past about could: it says that something is possible, or offers an option.

• He could come tomorrow, but he isn’t sure yet.
• We could go to the cinema tonight, if you like.

4. Could in conditionals and reported speech

Could is also the can of the second conditional and the can of reported speech:

• If I had more time, I could help you. (more on this in my guide to conditional sentences)
• «I can help you» → She said she could help me. (fully explained in the post on reported speech)

BE ABLE TO: the stand-in that fills the gaps

Be able to is not a modal verb: it is the verb be plus the adjective able plus a to infinitive. And precisely because it is not modal, it can do what can cannot: appear in every tense, in the infinitive, in the gerund and after another modal. If can is a specialist, be able to is the substitute who plays in every position.

Since it contains the verb be, it is conjugated like it. If that feels shaky, I have a whole guide to the verb to be.

The gaps it fills

That first case is exam gold: the future of can is will be able to, and there is no alternative. If you are revising future forms, have a look at my guide to will, going to and the present continuous.

When NOT to use be able to

And now the nuance almost nobody tells you: when you can use can or could, use them. In the present and past simple, be able to is correct but sounds formal, stiff and slightly odd in normal speech.

• Natural: I can swim. Stiff: I am able to swim.
• Natural: Can you help me? Odd: Are you able to help me?

To put it another way: be able to is an excellent substitute, but do not take the starter off the pitch.

Could vs was able to vs managed to

If you take one section away from this post, make it this one. It is the difference students ask about most, the one that comes up in the B2 First Use of English, and the one hardly any article explains properly.

The rule is this:

• General ability in the past (something you could do over a period): could or was able to, both work.
  I could run 10 km when I was younger. = I was able to run 10 km when I was younger.
• One specific achievement, on one occasion (you managed to do something difficult, once): NOT could. Use was/were able to or managed to.
  ✔ After three hours, the firefighters were able to put out the fire.
  ✔ After three hours, the firefighters managed to put out the fire.
  ✘ After three hours, the firefighters could put out the fire.

The trick for remembering it: could talks about capacity; was able to and managed to talk about result. If the sentence says that something worked out that time, you need the result.

Two exceptions worth knowing

In the negative the problem disappears. With couldn’t you can talk about general ability and about one occasion, with no restrictions:

• I couldn’t open the door. = I wasn’t able to open the door. Both are correct.

With verbs of perception and thinking, could does work even on one particular occasion: see, hear, smell, feel, taste, understand, remember.

• I could hear someone shouting in the street. ✔
• From the top, we could see the whole city. ✔

The difference at a glance

What can and can’t really sound like

This hardly ever appears in a grammar article and yet it is what makes people lose listening marks over a single vowel.

In a normal affirmative sentence, can is not pronounced /kæn/ but with a very weak vowel, /kən/, almost as if it were not there: I can swim sounds close to «I kn swim». Can’t, on the other hand, has a long, clear vowel in British English, /kɑːnt/, and the final t is often barely audible.

The practical conclusion for the exam: if you hear the long, strong vowel, it is negative. If the vowel slips past you, it is affirmative. I mention it because, in my experience as an exam preparer, half the mistakes with this verb in the listening paper are exactly that. 🙂

Common mistakes

These are the ones I see again and again. Read them slowly, because recognising them is half the battle.

Can, could and be able to in the Cambridge exams

This piece of grammar is examined from the very first level and never really lets go. This is what you will find.

A2 Key and B1 Preliminary

In the speaking test you will be asked about what you can do, and this verb is unavoidable: Can you play a musical instrument?, What sports can you do?. An A2 answer can be as simple as I can play the guitar, but I can’t read music. No fireworks needed: at that level, correct and clear is exactly what is being asked for.

B2 First: key word transformations

This is where the grammar turns into marks. In Part 4 of the Reading and Use of English paper you are given a sentence and a key word, and you have to rewrite it using between two and five words. ABLE and MANAGED are classic key words:

• They succeeded in finding a flat in the centre. ABLE
  They were able to find a flat in the centre.
• It won’t be possible for me to come on Friday. BE
  I won’t be able to come on Friday.

Notice that both need be able to rather than can: one is a past with a result, the other is a future. Exactly the two cases we saw above.

C1 Advanced and C2 Proficiency

At the higher levels the form itself is no longer what is tested, the nuance is: could have done for what was possible and did not happen (I could have told you, but you didn’t ask), or emphatic structures with inversion such as Not only could they finish on time, but they also…. At that level the examiner is not rewarding you for conjugating it, but for using it without thinking.

Summary table

Frequently asked questions

What is the difference between can and could?

Can is the present and could is the past, but could is also used in the present and future to talk about possibility and to ask for things more politely.

Can you say «I will can»?

No, never. Two modal verbs cannot follow one another. The future of can is will be able to.

Could or was able to for the past?

For a general ability, both work. For one specific achievement, only was able to or managed to. In the negative, couldn’t covers both cases.

Is be able to a modal verb?

No. It is be plus able plus an infinitive. That is why it is conjugated normally and can go where can cannot.

How is can’t pronounced in British English?

/kɑːnt/, with a long vowel, as opposed to affirmative can, which reduces to /kən/. That vowel difference is what tells you whether the sentence is affirmative or negative.

Can, could and be able to exercises

Now it’s your turn. Below you will find four interactive exercises. They mark themselves, so you will know straight away how you did. If you get one wrong, go back to the summary table and find the rule: that is the fastest way to make it stick.

And if you prefer a version in text that you can copy and paste, you have it down here 👇

Exercise 1: can, could or be able to?

Choose the correct option in each sentence.

Exercise 2: be able to in every tense

Complete the sentences with the correct form of be able to.

Exercise 3: right or wrong?

Decide whether each sentence is correct or incorrect, and rewrite the incorrect ones.

Exercise 4: key word transformations

Complete the second sentence so that it means the same as the first, using the word in capitals.

Conclusion

If you take three ideas away from all this, make them these: modal verbs never change their form and are followed by the infinitive without to; when the grammar calls for an infinitive, a gerund or a future, be able to takes over; and for a specific achievement in the past you say was able to or managed to, never could. That alone puts you ahead of most of the candidates whose work I mark.

This post opens a series on modal verbs, so over the next few weeks it will be the turn of must and have to, then should, then may and might. In the meantime, it is worth going back over the verb to be, which is what be able to is built on, and questions in English, where modal verbs have their own way of asking.

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See you in the next post. Until then, don’t forget to keep smiling! 🙂