What's the difference between before, until, and unless?

Question

Here are two fill-in-the-blank questions in my grammar exercises.

1. A man is not old ______ regrets take the place of dreams.

2. Success always hides behind the next corner in the road, so you'll never know how close it lies ______ you turn the corner.

When going through the questions, my classmates argued that before, until, and unless all sounds fine in the sentence. However, only one of them was considered correct, which I couldn't remember clearly.

Would anyone be so kind as to point me the correct answers out and show me why? Thanks in advance!

(*I'm not a native speaker, so sorry for any mistake. )

Answer 1

All three words would be grammatically correct, but not all grammatical sentences make sense.

Sentence (1) refers to the passage of time in a person's life, so unless is not appropriate. Until sounds much more idiomatic than before.

I think all three might be possible in Sentence (2). Unless you turn the corner implies that you might choose not to go round it at all. Assuming that you are going to continue along the road, until sounds better than before.