I don't understand when or why the "t" is added?

Bonjour Jenny ! In the case of inverted questions, you add the "t" for pronunciation reasons, whenever there are two vowel sounds clashing. This is explained in the related lesson: Forming inverted questions in Le Présent with il, elle, on However, thanks to you, I've now updated this lesson, by adding links to other lessons, as well as adding this explanation to it :) I hope that's helpful! Merci et à bientôt !