|be able + infinitive|
Diego Alenjandro from Colombia asks:
Is it OK to say: 'I haven't could do it'? What I mean is that I've been trying to do something, but actually I've not done it because I cannot. So, how can I say this?
Thanks for your attention, Roger. I hope I'm making myself clear enough.
Perfectly clear, Diego. The answer is that we can't combine this tense and this modal in this way. We must say either:
'I couldn't do it' or 'I haven't been able to do it'.
difference in usage is that if we say: 'I couldn't do it',
we are thinking about a particular action or actions that were completed
in the past, e.g.
if we say: 'I haven't been able to do it', we are thinking
of a period of time for the activity which extends right up to the
This is the sort of context that you are referring to, Diego, in the example you quote. Can has no perfect form, so we have to use has/have been able to.
Note that we can form the negative with not able or unable.
can has no future form either, so we must also use be able
to + infinitive when we want to refer to the future. Study the
also that we cannot combine can with another modal verb, so
if we want to use may, might or should and combine
possibility or probability with ability, we have
to use be able to and not can or could. Study
In the first two examples above, there is not much difference in
terms of possibility between may and might. They could
be used interchangeably without affecting the meaning. In the final
example above, it is likely or probable that the toilet will be
we normally use can or could in preference to be