Conditional Sentences

Conditional Sentences are about possibilities, not facts.
If I pass my exam, [then] I'll be happy.

When something is likely to happen, use the form :
IF [past tense clause] THEN [future tense].
If the bill arrives, I will pay it.

When something is unlikely to happen, use the form :
IF [past tense] THEN [conditional future].
If I was to win the lottery, I would pay the bill.

When discussing something that may have happened, had things been different, use :
IF [past perfect] THEN [conditional perfect].
If I had paid the bill, I wouldn't be in trouble now.

The conditional perfect never goes in the IF clause of the sentence.
If I'd have had is wrong.

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