Example:
* I didn't see you when you passed me in the street. If I'd seen you, of course I
would have said hello. (but I didn't see you)
* I decided to stay at home last night. I would have gone out if I hadn't been so
tired. (but I was tired)
* If he had been looking where he was going, he wouldn't have walked into the
wall. (but he wasn't looking)
* The view was wonderful. If I'd had a camera, I would have taken some photo-
graphs. (but I didn't have a camera)

Compare:
* I'm not hungry. If I was hungry, I would eat something. (now)
* I wasn't hungry. If I had been hungry, I would have eaten something. (past)

B. Do not use would in the if-part of the sentence. We use would in the other part
of the sentence:

* If I had seen you, I would have said hello. (not 'If I would have seen you')

Note that 'd can be would or had:
* If I'd seen you, (I'd seen = I had seen)
I'd have said hello. (I'd have said = I would have said)

C. We use had (done) in the same way after wish. I wish something had happened

= I am sorry that it didn't happen:
* I wish I'd known that Gary was ill. I would have gone to see him. (but I didn't know)
* I feel sick. I wish I hadn't eaten so much cake. (I ate too much cake)
* Do you wish you had studied science instead of languages? (you didn't study
science)
* The weather was cold while we were away. I wish it had been warmer.

Do not use would have... after wish in these sentences:
* I wish it had been warmer. (not 'I wish it would have been')

D. Compare would (do) and would have (done):

* If I had gone to the party last night, I would be tired now. (I am not tired now--
present)
If I had gone to the party last night, I would have met lots of people. (I didn't
meet?lots of people--past)

Compare would have, could have and might have:
* If the weather hadn't been so bad, we would have gone out.
* If the weather hadn't been so bad, we could have gone out. (= we would have
been able to go out)
* If the weather hadn't been so bad, we might have gone out. (= perhaps we
would have gone out)