If you asked someone who specialized in the Eleusinian mysteries, which involved the command "rain!" given to the sky by the participants, they would have had no trouble telling you how to say "it's raining". If you asked someone who had been reading the Iliad, they'd be able to tell you all kinds of words for spears, fighting, armor, walls, and viscera, but rain might not come up as frequently.
This question is basically just "what's the verb for 'rain'?". The syntax is exactly what you'd expect.
As it happens, the word is very well documented ( http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%... ), but it's not obvious that your TA should have been reading about rain. You know the words you use frequently.
If you asked someone who specialized in the Eleusinian mysteries, which involved the command "rain!" given to the sky by the participants, they would have had no trouble telling you how to say "it's raining". If you asked someone who had been reading the Iliad, they'd be able to tell you all kinds of words for spears, fighting, armor, walls, and viscera, but rain might not come up as frequently.