be able + infinitive

'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.

• 'I couldn't repair the car by myself, so I asked a mechanic to help me.'
• OR: 'I wasn't able to repair the car by myself, so I asked a mechanic to help me.'
• OR: 'I was unable to repair the car by myself, so I asked a mechanic to help me.'

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:

• 'I've worked on it every day this week, but I still haven't been able to complete the report.'
• 'They've been unable to visit him since he was admitted to hospital.'
• 'Have you been able to find out anything about her?'

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:

• 'I have a very poor sense of balance, so I don't think I shall ever be able to ride a horse.'
• 'He is badly injured, that's true, but I'm sure he'll be able to walk again by the summer.'
• 'If the snow continues to fall, we'll be unable to leave the house.'

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:

• in the sense of know how to
• with verbs of the senses such as hear, see, smell, feel, taste
• with verbs of thinking, e.g. decide, remember, understand, believe.

Study the following:

• 'Can you speak Japanese?'
• 'Can you see what it says on the departures board?'
• 'I can't see a thing without my glasses.'
• 'I couldn't taste the garlic in the mayonnaise, although my wife could.'
• 'I can't remember when I last saw Joan.'
• 'I can't believe you're going to marry him.'
• 'They couldn't decide whether to buy a red or a blue car.'