Hypothetical sentences and what tense should I use with them?

If I talk about something hypothetically, like having a test and failing it, should I say:

1) If we had a test on prepositions right now, I'd get a D.

2)If we would have a test on prepositions right now, I'd get a D.

Another example:

Hypothetical person I talk about is dead, so which of these should I say:

3) If he would have been interested in Physics instead of Biology, he would have made big contribution(s) to Physics.

4) If he would had been interested in Physics instead of Biology, he would have made big contribution(s) to Physics.

Or should I change the sentence completely:

5) Making sure that he was more interested in Physics than in Biology to begin with, he would have been as good at/ in it.

Which of these/ those sentences are right?

Many thanks, l.