CAN, COULD AND (BE) ABLE TO A We use can to say that something is possible or allowed, 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 hotel.
The negative is can’t (= cannot): I’m afraid I can’t come to the party on Friday.
B You can say that somebody is able to do something, but can is more usual: We are able to see the lake from our hotel.
But can has only two forms: can (present) and could (past). So sometimes it is necessary to
use (be) able to.
Compare:
I can’t sleep. I haven’t been able to sleep recently.
Tom can come tomorrow. Tom might be able to come tomorrow.
CA We had a lovely room in the hotel. We could see the lake.
As soon as I walked into the room, I could smell gas.
D Could and was able to We use could for general ability and with see, hear etc. :
My grandfather could speak five languages. I could see them, but not very clearly. The negative couldn’t (could not) is possible in all situations:
My grandfather couldn’t swim.
MUST AND CAN’T We use must to say that we believe something is certain: You’ve been travelling all day. You must be tired. (travelling is tiring and you’ve been travelling all day, so you must be tired) We use can’t to say that we believe something is not possible: You’ve just had lunch. You can’t be hungry already.
(we don’t expect people to be hungry immediately aft er a meal)
The structure is:
you/she/they (etc.)
must
can’t
be (tired / hungry / at work etc.)
be -ing (doing / going / joking etc.)
get / know / have etc.
