French language question: when to use de vs. de la/du/de l'/des?

When I was reading something in French cl today, I noticed a sentence that goes, "Il pose des questions � son professeur d'histoire sur la question de l'identit� des Fran�ais." Can someone explain to me why it's des questions instead of des les questions, d'histoire instead of de l'histoire, and de l'identit� but not d'identit�?