Can, could and be able to

When you use it

You use can, could and be able to to talk about ability: what someone knows how to do, or what is possible for them in a situation. Can is for the present (I can swim), as you saw in Can and can't. For the past, English has could, but also was able to and managed to, and the choice depends on one question: are you talking about a general ability, or about one success on one occasion?

Can has only two forms, can and could. It has no infinitive, no -ing form and no participle, so it cannot follow will, have, want to or a preposition. In all these places English uses be able to instead.

Could has other meanings too. For a guess about what is possible (It could rain later), see Might and may and Modals of deduction. This lesson is only about ability.

How it is formed

Able is an adjective, so it usually needs a form of be before it and to after it: I am able to, she was able to. Can and could never take to.

The rule for the past is the heart of this lesson. Could describes an ability someone had over a period of time: When I was a child, I could climb any tree. For one particular success, positive could usually sounds wrong to most speakers, so use was able to or managed to: The last bus had gone, but we were able to get a lift with a neighbour. Managed to adds the idea that it was difficult. In negative sentences there is no such problem: couldn't and wasn't able to are both fine for one failure.

There is one exception. With verbs of the senses and of the mind, such as see, hear, smell, feel, understand and remember, could is normal even for one moment: The moment the plane landed, I could see snow on the hills.

For the future, will be able to is the usual form for an ability you do not have yet. When you arrange something now for later, can is also possible: I can meet you tomorrow.

Examples

• Anna can speak Polish and Italian, but she can't drive.
• When Tom was young, he could run very fast.
• Anna couldn't ride a bike until she was ten.
• The road was closed, but we were able to find another way.
• After many tries, Tom managed to fix the old radio.
• In the dark room I could hear someone breathing.
• Anna will be able to drive alone after her test in June.
• I haven't been able to contact Tom since Monday.
• You need to be able to type quickly in this job.
• Being able to relax after work is important.

Common mistakes

• ✗Tom will can help you next week.

  ✓ Tom will be able to help you next week.

  Can has no infinitive, so it cannot follow will. Use will be able to, or can alone when you are arranging something now. Ukrainian has one word for this, зможе, so learners look for one English word too, but English needs will be able to. Russian has one word for this, сможет, so learners look for one English word too, but English needs will be able to. German says Er wird dir helfen können, with können as an infinitive. English can has no infinitive, so be able to takes its place.

• ✗The exam was hard, but I could pass it.

  ✓ The exam was hard, but I was able to pass it. / I managed to pass it.

  This is one success on one occasion, so positive could does not fit. Could is for a general ability in the past. Ukrainian already makes a similar difference: міг is close to could, and зміг is close to was able to or managed to. Russian already makes a similar difference: мог is close to could, and смог is close to was able to or managed to. German konnte covers both meanings (Ich konnte die Prüfung bestehen). In English, choose was able to or managed to, which is like Es ist mir gelungen.

• ✗I have could not sleep since Sunday.

  ✓ I haven't been able to sleep since Sunday.

  Can has no past participle, so the present perfect uses have been able to. Ukrainian uses the present tense here, не можу спати з неділі, but English needs the present perfect with since. Russian uses the present tense here, не могу спать с воскресенья, but English needs the present perfect with since. German can say Ich kann seit Sonntag nicht schlafen in the present. English needs haven't been able to with since.

• ✗I want to can play the guitar.

  ✓ I want to be able to play the guitar.

  After verbs like want, hope, need and would like you need an infinitive, and can has none. Ukrainian хочу вміти uses an infinitive of вміти. English can has no infinitive, so use to be able to. Russian хочу уметь uses an infinitive of уметь. English can has no infinitive, so use to be able to. German puts können at the end: Ich möchte Gitarre spielen können. English puts to be able to straight after want.

• ✗Anna can to swim. / ✗ Anna is able swim.

  ✓ Anna can swim. / Anna is able to swim.

  Can is followed by the base verb with no to, but able always needs to. Ukrainian може плавати has no word between the two verbs, so the to after able is easy to forget. Russian может плавать has no word between the two verbs, so the to after able is easy to forget. German works the same way: Sie kann schwimmen has no zu, but Sie ist in der Lage zu schwimmen has one, like able to.

Practice

The exercises go from easy to harder. Each answer is checked at once. If it is wrong, you get a hint and a second try.