be able to do用法(Roger Woodham)

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'.

The 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.

However, 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 present, e.g:

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.

Similarly, 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 following examples:

Note 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 the following:

  • 'The doctor might be able to see you this afternoon.'
  • 'This store is closing, but we may be able to offer the sales staff a job in another branch.'
  • 'I should be able to fix the upstairs toilet with the tools in your toolbox.'

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 fixed.

Finally, we normally use can or could in preference to be able to:

Study the following: