Conditional could, would be able to

When could (+ present infinitive) is used as the tentatibe form of can, it refers to present or future time :   a I could do it for you now if you like.   b I can't do it immediately, but I would do it tomorrow morning. The reported version of sentence b is :   I told him I couldn't do it immediately, but that I could do   it the following morning. In conditional sentences, could very often represents the 'unreal' present:   If I knew how it worked, I could tell him what to do.   (= but I don't know, so I can't tell him.) The equivalent form of be able in this case is would be able (not, of course, was able). In a conditional sentence, could + perfect infinitive expresses unreal past:   If I had known how it worked, I could have told him what to do.   (= but I didn't know, so I couldn't tell him.)