A question about the conditional construction

[359158666]

Hi
Recently, I reviewed my Manhattan SC note. I found that in chapter 6 the author list 5 examples of the conditional construction. As to a past condition, the construction should be " If sth had done, sth would have do" However, on the GMAT OG 13 SC 69, the explanation said that for a past condition, the main clause should be " If X happened, Y would happen". I don't know which is correct. Can you give me a more detailed explain. Thank you

[RonPurewal]

If sth had done, sth would have do[ne]

This construction is for something hypothetical in the past. I.e., we're wondering about an alternate version of some past situation.
E.g., If the Cold War had escalated into nuclear bombing, all of us would have been exposed to radiation.

If X happened, Y would happen

This is for a hypothetical version of something right now.
E.g., If I were in Moscow right now, I would go see my friend Pavel.
(I.e., I'm not in Moscow right now.)

[RonPurewal]

By the way, if you are talking about a situation that hasn't happened yet but can reasonably still happen, then there's a third construction:
If I see those tomatoes at the store, I will buy them for you.

You could also write If I saw..., then I would buy..., but that version indicates doubt that the tomatoes will be at the store. The previous version implies that the tomatoes are reasonably likely to be there.