That's not that the final letter is silent, it's awfully explained (although this course is good), it is that in French, to make a sound, you must have a consonant + a vowel. So the final letter is part of the sound. "g" has not sound alone, but "ge", "gi", "go","ga" makes a sounds. So in "garage", it's "ga-ra-ge". The "e" is not silent, it's pronounced with the "g".

This lesson is not good. You can pronounce "lentement" 2 ways, slowly: len-te-men or quickly: lent-men. Sous le bureau= the "e" is heard.

Appartement: spelling mistake.

When you talk correctly "je te dis", the e is not silent. If you talk quickly, it can be.