Can and be able to
In the present tense, be able to is a little more formal and less usual than can:
Emma is good at computers. She can write/is able to write programs.
However, in some structures we always use be able to,not can.
To-infinitive: It's nice to be able to go to the opera, (not to can go.
After a modal verb: Melanie might be able to help us.
Present perfect: It's been quiet today. I’ve been able to get some work done.
For the future we use can or will be able to, but NOT will can
If we earn some money, we can go/we'll be able to go on holiday next summer.
I'm afraid I can't come/I won't be able to come to the disco on Friday.
But to suggest a possible future action, we normally use can.
Let's have lunch together. We can go to that new restaurant
Could and was/were able to
For ability or opportunity in the past, we use could or was/were able to.
Natasha could play (or was able to play) the piano when she was four.
In those days we had a car, so we could travel (were able to travel) very easily.
In negative sentences and questions, we can use either form.
It was foggy, so the plane couldn't/wasn't able to take off.
The pool was closed, so they couldn't/weren't able to have a swim.
Could you/Were you able to describe the man to the police?
Exercise III. Use can or can΄t.
1. ___ You speak English?
Yes, I ___ but I___ speak Greek
2. ___I buy a new coat today?
No, you __.
3. Why ___ we go shopping to day?
Because, the stores are closed.
4. I am sorry. You ___ take that suitcase on the plane.
5.___ Barbara carry that big box?
No she ___ but her husband, Charles___
6. Whom ___ you see from your office window?
I __ see the bank and I ___see the bus stop.
7. Why ___ you come on Wednesday?
Because we are going to be busy.
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