Past Simple+ would/could/might
Incorrect : If he would ask me, I would Stay.
Correct : If he asked me, I would stay.
Incorrect : If I would be able, I could help him.
Correct : If I were able, I could help him.
Note: An ‘improbable’ condition referring to an unrealistic future action is expressed by the past simple tense and in this case the principal clause contains would, could or might followed by a simple bare infinitive. As a matter of fact, the past tense here does not indicate any time but a very low degree of probability. The action referred to in this kind of conditional sentence is very unlikely to happen.