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1. you have to know the whole story to understand. (you need o know the whole story,,,)2. you'd have to know the whole story to understand.
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#2 is hypothetical.

A knows the whole story.
B dos not know the whole story. He does not understand.


A: You would have to know the whole story to understand. = If you knew the whole story, you would understand.

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Hasibul Alam ... confused about 2nd one

It's an implicit second conditional.

You'd have to know the whole story to understand.

~ You would have to know the whole story in order to understand.
~ You would have to know the whole story if you wanted to understand.

CJ