"The dog stands."
Yea :) I've been trying to work backwards too, with words I don't know.
It's the verb stem + ㅂ니다 if the stem end in a vowel; like 서다 -> 섭니다 as you wrote about.
And verb stem + 습니다 if the stem ends in consonant. 먹다 -> 먹습니다 I'm sure you already know, but I'm just writing it here for reference :)
Not always. Sometimes it's 이/가 (subject marking particle), other times it's 은/는 (topic marking particle). From what I've managed to pick up I think 이/가 is used if you want to emphasize who is doing the action while 은/는 is more like emphasizing what/who you're talking about.