1. Would be like someone saying "If I ever saw you at some time in the future I would greet you." Therefore, it is possible for me to see you in the future and greet you. Similar Example: "If I was walking down the street and I saw you, I would say hello!" 2. Would be like saying "If I had seen you in the past I would have greeted you" (but since I didn't see you, I couldn't have greeted you) It is impossible to go back in time and greet you at that same time and place. Similar Example: "I did not see you at the party but if I had seen you I would have said hello!" Hope this is helpful and easy to understand!

I think it is not your level of English. You should learn the basics before asking such a question. For example "What ARE the differences" because the word differences is PLURAL. I would really help you but I think you would not understand the explanation. Anyway here are some explanations: http://www.englisch-hilfen.de/en/grammar/if_type1.htm http://www.englisch-hilfen.de/en/grammar/if_type2.htm http://www.englisch-hilfen.de/en/grammar/if_type3.htm

2 actually, I had NOT seen you, and I have NOT greeted you. but as the regular above, If I had seen you, I would had greeted you.

Well, both of the are correct, but there is a LOGICAL difference: The first sentence says that if I ever see you passing me by on the street, I would greet you. For the second one consider the story: you see me on the street and you say "I saw you yesterday in the park and I said hello. Why didn't you greet me back?". I obviously would have greeted you if you had seen you, because we are friends, but I haven't seen you, so I would say "If I had seen you, I would have greeted you.". There is obvious grammatical difference between those two sentences, but I think it is easier to distinguish them this way :D

1 indicative mood 2 subjunctive mood 1 to express a regular. 2 to express a pity or regret