Modern English, apart from a few vestiges, does not express mood by morphology: instead, it has a range of modals (such as will, can, must, should) each of which has its own range of meanings.
Some of them can be used to translate particular morphological constructions in other languages: but in my view nothing is gained by trying to put labels on the ...
It makes no difference.
He could have passed, with harder study.
He would have been able to pass, with harder study.
He didn't study hard, so he was not able to pass.
He didn't study hard, so it was not possible for him to pass.