Why they sing is an indirect question, here used as the principal term of a prepositional phrase.
The verb in a direct question is in the indicative mood, but the mood is subjunctive in an indirect question.
So, if we report a question instead of asking it directly, we have an indirect question.
In an indirect question the verb is in the subjunctive and its tense is determined by the law for tense sequence.
The above list will hopefully provide you with a few useful examples demonstrating the appropriate usage of "indirect question" in a variety of sentences. We hope that you will now be able to make sentences using this group of words.