When did you get Andy Lau's autograph?

-when I was 15 years old.

Since I am 15 years old, that is to say, since I am 15 years old.

This phrase means that it is difficult to do something. It means "it is difficult to do something", so it is right to fill in finishing.

If is generally only used to guide object clauses, while when can guide four noun clauses. If and or not can't use when.

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