Modals

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1. Can

Can + infinitive (can speak)

Can has only two forms - present (can) and past (could)

We use can to say that something is possible/somebody has the ability to do something

Can you speak any foreign language?

I can come and see you tomorrow if you like.

2. (be) able to

it is possible to use it instead of can, but can is more usual

as can has only two forms, sometimes it is necessary to use (be) able to :

I can´t sleep.

I haven´t been able to sleep recently. (can doesn´t have present perfect)

3. could

Sometimes could is the past of can. We use could especially with: see, hear, smell, taste, feel, remember, understand

When we went into the house, we could smell burning.

We also use could to say that somebody has the general ability or permission to do something

My grandfather could speak five languages.

We were completely free. We could do what we wanted.

We use could to talk about possible actions now or in the future/suggestions (we can use can, but could is less sure than can)

It´s a nice day. We could go for a walk.

The phone is ringing. It could(=might) be Tim.

I am so tired. I could sleep for a week. (now)

I was so tired. I could have slept for a week. (past)

More often we use could have (done) for things which were possible but didn´t happen

Jack fell of a ladder yesterday but he is all right. He is lucky - he could have hurt himself badly. (but he didn´t hurt himself)

!!! We use could for general ability. But if we are talking about what happened in a particular situation, we use was/were able to.../managed to...

The fire spread through the building quickly but everybody was able to escape.

Jack was an excellent tennis player. He could beat anybody.

Jack and Alf had a game of tennis yesterday. Alf played very well but in the end Jack managed to beat him.