Translation:Not even after twenty years could he forget about the woman?
This is a tricky one! The main translation -- Not even after twenty years could he forget about the woman? -- definitely sounds odd as a QUESTION. On the other hand, the grammatically correct option that vor23 suggests -- Could he not forget about the woman even after twenty years?"-- sounds strange, too, and I wouldn't expect to hear anyone actually use it.
So, since I haven't been able to think of anything that sounds better, I think I'd vote with HughLomas and lobachevsky for "Even after twenty years he could not forget about the woman?" While it's clearly not a standard question construction, the sentence doesn't seem to lend itself to a standard construction!
I am replacing this exercise with a statement version. The inversion may sound stilted to some, but it is possible, and if the character is so messed up by whoever the woman was, the narrator could be forgiven for a little drama. Also, the reverse and third-language takers may glean something new from the inverted structure in the English statement.