Can, Could, Be able to, Be allowed to
can, could and be able to
be able to
it can be used instead of can or could
when we talk about sth that is happening as we speak
- Watch me, Mum! I can stand on one leg. (not I'm able to)
before passives
- CDs can now be copied easily. (not are now able to)
when the meaning is "know how to"
- Can you cook? (not Are you able to)
be able to rather than could
single achievements rather than general ability
- She swam strongly and was able to cross the river easily. (not could cross)
could is more natural
in negative sentences
with verbs of the senses and thinking
(feel, hear, see, smell, taste)
(believe, decide, remember, understand)
- I could remember the crash but nothing after that. (rather than I was able to remember)
after phrases the only thing/place/time and after all when it means "the only thing"
- All we could see were his feet. (rather than All we were able to see)
to suggest that sth almost didn't happen
(almost, hardly, just, nearly)
- I could nearly touch the celling. (rather than I was nearly able to touch)
can and could
talking about possibility
could not can
theoretical possibility
- It could be expensive to keep a cat. (if we had one, it could or it may not be expensive)
can not could
when sth is possible and actually happens
- It can be expensive to keep a cat. (it can and sometimes it is)
can't not couldn't
when sth is theoretically or actually impossible
- There can't be many people in the world who haven't watched television.
- The doctor can't see you this morning.
can
where there is a very real possibility of a future event happening
- W can stau with Jim in Oslo. (we will be able to stay)
could
to suggest that sth is less likely or there is some doubt
- We could stay with Jim in Oslo. (it's posible if he's there)
could and be allowed to
talking about permission
could or be allowed to
for general permission in the past
- Anyone could fish in the lake when the council owned it.
- Anyone was allowed to fish in the lake when the council owned it.
be allowed to but not could
one-off permission
- Although we didn't haver a ticket, Ken was allowed to come in. (not could come in)
in negative sentences both are ok
- We weren't allowed to open the presents until Chrismas morning.
- We couldn't open the presents until Chrismas morning.
could and can + have + past participle
could have + past participle
when sth was technically possible but didn't happen
- You could have taken a taxi from the airport. (but you didn't)
why not could alone?
can have + past participle
to express probability of sth acording to us
can have + past participle
can have + past participle
questions
- Can he have forgotten about our appointment?
negations
- I can’t have lost my keys again!
negating words
(barely, hardly, only, or never)
- I can hardly have been responsible for your losing your keys.
- I tried to get up but I couldn't move. (rather than I wasn't able to)
with subordinate clouse
- He can have fotgotten about me.
- I don't believe he can have forgotten about me.