Questions about example sentences with, and the definition and usage of "Weakening"

A: The second example alludes to a comparison. The first example is correct.

The second one should be like this: If it had not been the weakening of the dollar, it would have been my lack of savings that meant I could not buy a car.

A: it has no meaning，it is just a word like an “empty subjuect”.English sentence must have a predicate and a subject so we must use it to make it a right sentence.

A: In this context "for" means "because." Without the "for" the clause that comes after the ellipses is different. For example,

"If it had not been for the weakening of the US dollar, I could buy a new car!"
"If it had not been the weakening of the US dollar, then it had been the market crash."

Without the "for" you're implying that there's another reason besides "the weakening of the US dollar"

With the "for" you're saying that the weakening of the US dollar is the reason.

A: The meaning of each is ever so slightly different. Number two is probably the easiest to use. It depends on exactly what you were trying to convey.