I wrote "We watch the interesting movie together". I still get confused as to when is right to use "the" or "a(n)"
From what I know, context is what decides. But I think the interesting movie should be correct too since it's not technically incorrect.
If the word after "a" starts with a vowel, "a" becomes "an." "A letter is in an envelopen"
The curt answer is "Because that's what the page says." The honest answer is "Who Knows". Don't ask "why" when it comes to how something is said. My DLI instructor said that the way the book says is the most common way to say something.