Unit 26. Can, could and (b) able to

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

We use can + infinitive (can do / can see etc.)

• We can see the lake from our bedroom window.
• Can you speak any foreign language?
• I can come and see you tomorrow if you like.

(Be) able to... is possible instead of can, but can is more usual.

• Are you able to speak any foreign languages?

But can has only two form, can (present) and could (past). So sometimes it is necessary to use (be) able to..

• I can't sleep.
  I haven't been able to sleep recently. (can has no present perfect)
• Tom can come tomorrow.
  Tome might be able to come tomorrow. (can has no infinitive)

Could and was able to..

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.
• She spoke in a very low voice, but I could understand what she said.

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

• My grandfather could speak five languages.
• We were completely free. We could do what we wanted.