Sorry if my off-hand comment hurt you, Nocker. Didn't mean any harm with it - it was just, at face value, your initial question sounded like "how do I describe things in a roleplaying game?", which just seemed strange. No hard feelings, I hope.
Thing is, I don't think there is really a rigid conversation structure (there's an emergent one based on MC principles and player moves that ask questions, but that's different).
So the thing you mentioned (when the room goes silent and everyone looks to you...) to me seems not like a rule that says when an MC gets to talk, but a prompt in case of an awkward scenario. It's a reminder of the tools you have available, when there is a lull in game activity and the players look at you expectantly. If you already know how to fill that silence, you don't need the reminder.
As for when you use a move and when you don't... Maybe it's worth looking at it a bit differently? Like Christopher says, a MC is in a conversation, is "just describing" all of the time. Some of the times, what you're describing also happens to be a move, but not necessarily so.
Now, most of the stuff you'll be saying will in fact be moves, I'm sure - you use move when things happen and things happen often in AW. But moves are not primary in the way talking about what's happening in the fiction is.
So you don't use moves and not-moves, you just talk and sometimes use moves while talking.
Edit: A bit of this is a cross-post with noclue. Sorry for any redundancies.