Translation:I wonder what he did in the seventies.
You turned it into a question. It'd be something like; I wonder: What did he do in the seventies?
You keep the same sentence structure in statements (I wonder what he did in the seventies).
Statement - I don't know what that is
Question - What is that?
There are a few exceptions, but that is the general gist of statements. For example, you'd say something like "there is a cat that is orange"; you wouldn't say "there is a cat that orange is".