Luckily, I don't recall having to use would vs. will as a decision point on an official SC question, so I wouldn't worry too much about this!

The short answer is that we use "will" for things that are real possibilities and "would" for things that are hypothetical or imagined. Here, have an example:

"If I had a billion dollars, I would buy you a pretzel factory."

The use of "would" implies that I will likely never actually have a billion dollars, and that this is just a fantasy (a hypothetical situation). In this case, I don't believe that I will ever actually buy you a pretzel factory, sadly.

"Once I make my first million dollars, I will buy you a really large pretzel."

The use of "will" implies that this is a very REAL possibility. Note that that does NOT mean it's DEFINITELY going to happen. But in this case, I believe that I will make over a million dollars and that I will be able to buy you a really large pretzel someday.

So it's not about knowing with certainty whether something is going to happen. It's about whether the possibility is in fact realistic or simply imagined/fantasy/hypothetical. Of course, that distinction can be very subtle and subjective, and that's part of why we don't see this as a common decision point on the GMAT.

In short, as long as it's something that you think could possibly happen (even if you don't KNOW that it will happen), then "will" might be justifiable.

I hope that helps!
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