A few hopefully related answers:
agelessdrifter wrote:1) I find myself lacking expressions to convey humility. Say for instance I got a 100 on a test and someone says "いい学生だね。” or like "頭がいい” or something, what's the appropriate response? Like if someone says "Oh, you're a good student" or "You must be very smart" I don't want to go full bore into "No, I'm not a good student" or "No, I'm an idiot" unless that's the normal thing to say. In the states I'd say something like "I just have lots of time to study" or something, but I'm afraid that in my broken Japanese it might just sound like "I study a lot" which would come across more like bragging. I know how to wave off complements on my Japanese speaking with, eg, まだまだです or いいえ、あまり上手じゃありません, but are there any other good stock expressions to use in more generic circumstances?
There's no one way of expressing this (as it shouldn't surprise you to hear). A couple of good general expressions would be:
そんなこと（は）ありません。 - That's not so.
そうでもない。 - Not really.
2) What's the appropriate valediction when leaving a store or restaurant? I know さようなら is only really used infrequently and has a sort of permanent connotation, similar to goodbye. じゃ、また seems too familiar or friendly but is my best guess. I feel weird being told "thanks for coming in, goodbye, goodnight" and whatever else is being said to me as I walk away from a cash register and just, like, nodding and waving.
If it's a restaurant then 「ご馳走様でした ＜ごちそうさまでした＞」 is appropriate. Otherwise, just 「ありがとうございます。」
3) Is there a way to use そうです to express that something looks to have already occurred? My understanding is that, for instance,
雨が降たそうです。 would mean you heard that rain had fallen (or heard it falling, even?) while
雨が降りそうです。 would mean that rain appears (specifically, looks) to be about to fall
but is there a way to convey something like "it looks like rain fell here," using this form?
「Verb + そうです」, as you correctly state, means that you heard something, in the sense that you were told it.
So 「雨が降ったそうです。」 means "I heard that it rained," as in that you heard it on the news, or a friend told you, for example. This construction can be used with verbs in any tense, and even with other parts of speech: 明日は天気が悪いそうです。 ＜あしたは てんきが わるいそうです＞ - I heard that the weather will be bad tomorrow.
「Verb stem + そうです」 means "it appears that," or, "it seems that," something is about to happen, or is likely to happen. So not specifically 'looks'. You could use 「雨が降りそうです」 if the sky was overcast, but also if you heard thunder. For that matter, you could have got the information from a weather chart. The core implication is that you're drawing a conclusion about the (relatively) near future, with some fairly clear evidence. Be warned that there are other, somewhat similar meanings for this construction as well.
The formation you're looking for for, "it looks like rain fell here," is 雨が降ったよう
です。 This implies a conclusion drawn, from some evidence, and can refer to past, present or future states.
There is, as you'd expect, some overlap between these three, and which is more natural/correct is somewhat situational.
Sticking with weather examples:
来週は寒くなるそうです。 - It's going to be cold next week (I heard them say so on the weather forecast)
来週は寒くなりそうです。 - It looks like it's going to be cold next week (They're forecasting strong northerly winds and snow)
来週は寒くなるようです。 - It'll probably be cold next week (Based on meteorological evidence) - This is what they usually say on
the weather forecast.
I hope this is coherent, as it's getting a bit late.