Can you help me with this homework question?

What the question gives me is this:

L' ? (tu)

Non, je ne pouvais pas (could not) dire qui il était.

I need to fill in the blank to form the question that goes with the given answer.

Here is an example of a completed one:

Sais-tu que j'ai une nouvelle maison?

Oui, on m'a dit que tu as une nouvelle maison.

Answer

If the L' isn't absolutely necessary, here is what you should do:

Non, je ne pouvais pas (could not) dire qui il était. --> The verb here is "pouvoir" and what completes the sentence is "dire qui il était". You need to start your question with the verb in the present perfect (passé composé) ( http://www.200words-a-day.com/french-verb-pouvoir.html ) and your subject afterwords, following with the rest of the sentence.

So basically your sentence should follow this pattern :

Auxiliary "avoir" in the present tense + subject "tu" + verb "pouvoir" in the right form (pu) + rest of the sentence.