Originally Posted by peteryoung
1) If you had paid me, I would have mowed the lawn. - It's no longer possible to mow the lawn. The time in which you could have mowed the lawn has passed, and you didn't do it because that cheapskate didn't wanna pay you. This is in the past. The result can't be changed. He/she didn't want to pay you, and you didn't mow the lawn - done. - imaginary in the past
2) If you paid me, I would mow the lawn. - The cheapskate doesn't want to pay you. Therefore, you are not going to mow the lawn. It is unlikely that he/she will pay you, and unlikely that you will mow the lawn. This sentence speaks about something possible but unlikely in the present. This situation can still change. It's not finished. The statement speaks of something that seems to be unlikely, but it is still possible. - imaginary in the present
3) If you pay me, I will mow the lawn. - I'm making you an offer. Tell me you'll pay me, and I'll mow the lawn. I'll mow the lawn, but you have to pay me. The speaker sees this idea as a real possibility. This is in contrast to sentence number 2, which does not speak of the same idea as a real possibility. - real possibility in the present
Sentence number 3 is more optimistic than sentence number 2.