Study this example situation:

Last month Gary was in hospital for a few days. Rachel didn't know this, so she didn't go to visit him. They met a few days ago.
Rachel said:

If I'd known you were in hospital, I would have gone to see you.

Rachel said: If I'd known (= If I had known) you were in hospital. This tells us that she didn't know.

• 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 didn't go out 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 with me, I would have taken some pictures. (but I didn't have a camera)

• 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)

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

• If I'd seen you, (I'd seen = I had seen)
• I'd have said hello. (I'd have said = I would have said)

• 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'd studied science instead of languages? (you didn't study science)

• The weather was cold when we were on holiday. I wish it had been warmer. (not I wish it would have been)

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