He said "it's only when I have too many drinks that I feel like hell in the morning " and last night he had too many drinks. So why doesn't he feel like hell this morning?

This is an example of an "only if" problem.

"Only if A then B" can be rewritten as:
If B then A, which be rewritten as: If not A then not B. These are the only conclusions that can be drawn.

The relevant statement is "only when I have too many drinks that I feel like hell in the morning".

So you can draw a conclusion that "If I feel like hell then I had too many drinks"; and also "If I didn't have too many drinks, then I don't feel like hell".

But you cannot rewrite / or conclude it as "if I had too many drinks, then I feel like hell in the morning". "Only" makes a big difference.

Remember these 3 conditional scenarios:
1. If A then B, you can conclude if not B then not A
2. Only if A then B, you can conclude if B then A, and by rule 1: if not A then not B
3. If and only if A then B, you can conclude if A then B, and if B then A

Google "LSAT only if", there are many free explanations.