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: https://progress.lawlessfrench.com/revision/grammar/forming-questions-by-inverting-in-the-present-tense-il-elle-on-forms 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 !