If clause type 3 is used if the condition is impossible to happen because it has happened in the past
If he hadn't eaten the fish, he would have survived. ==> But he has eaten the fish....bad luck!
An "conditional" or "if -clause" has two parts: The "If part" and the "main" part.
If part
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Main part
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If I had learned for my exams,
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I would have passed the 7th grade.
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If + Personal pronun + Past Perfect + rest
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Personal Pronoun + woulc + have + Past Participle+ rest.
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You can swap the if-part and the main part:
If he hadn't eaten the fish, he would have survived.
OR
He would have survived if he hadn't eaten the fish.
But: If your "If-part" is first, you must put a comma behind it.Remember: The if works like a comma in If-clauses.