Are you asking about the conditionals? -If I studied hard, I would pass the exam. (here you've still got the chance to study hard and pass the exam) - If I had studied hard, I would have passed the exam. (the 3rd conditional refers to the PAST, so now there's no more chance to study for that exam, because you already did it )

Type 1: If I HAVE time, I WILL visit you. (Maybe I will have time. I do not know at this moment.) Type 2: If I HAD time (I do not have time, for I am always working), I WOULD visit you. (So I am not going to visit you. Sorry.) Type 3. If I HAD HAD time last week (I did not, for I had to work), I WOULD HAVE VISITED you. (So I did not visit you last week. Please forgive me.) ***** If I have $100, I will lend it to you. Let me check my wallet right now. If I had $100, I would lend it to you. But I cannot lend it, for I do not have $100. If I had had $100 last week, I would have lent it to you. But last week, I did not have $100, so that is the reason that I did not lend you the money. Please don't be angry with me.

i see;) thansk for your explain :)

Like Maria said, only Type 3 is used to re-imagine the past. Type 2 can't be used for the past, as it only imagines a different present, or an unlikely future.