subjunctive mood - The third conditional for "if I could"

What is the third conditional for "if I could"? For example, we say:

If I had studied hard, I would have passed the exam. How about this:

If I could study, I would have passed the exam.[Is there a past perfect for could?]

Answer

Although could is the past tense of can, past tense in modals often doesn't mean past time. In the if I could, the past tense could has a counterfactual meaning rather than a past time meaning. Of course, the verb following the modal can't be past tense, so to give it a past time meaning, use the perfect aspect (i.e., could + have + past participle). So your example about studying could be changed to

If I could have studied, I would have passed.

Another option, as Irene has pointed out, is to avoid could and use be able, leading to

If I had been able to study, I would have passed.

The idea of zero, first, second, and third conditional has been much criticszed. See, for example, here.