Translation of basic sentence confused on subject placement

Question

I am attempting to translate some sentence and got some weird solutions.

1) There is a post office over there. There is a bank, too.
あそこはゆうびんきょくですあそこもぎんこうです。

I guess what I am saying is there (over there) is a post office. Apparently the correct solution is あそこにゆうびんきょくあります。あそこにもぎんこうあります。I am assuming in this context "there" is a place why can't I use wa?

2) There's professor yamashita in front of high school students.

Why can't I start this sentence with やましたせんせいは isn't he the subject of the sentence? I think one has to write こうこうせいのまえにやましたせんせいがいます。but then I have no idea what the subject of the sentence is?

Answer 1

1. You're missing a particle in the "correct" answers:

   あそこにゆうびんきょくがあります。

   The particle に marks the location in which something exists. The particle が marks the subject.

2. Professor Yamashita is the subject of the sentence. In Japanese, when the subject appears in the sentence (and very often it doesn't!), it can be marked with が or は, depending on the context. Because Prof. Yamashita is marked with が in this sentence, it implies that you weren't already talking about him; i.e., he's not the existing topic (which is marked with は). Here's a valid, if stilted, conversation:

   Q: やましたせんせいはどこですか。
   A: やましたせんせいはこうこうせいのまえにいます。

   I say stilted because you wouldn't really repeat the topic in the answer, although in a language class you're obviously expected to follow particular forms. Here's a somewhat more natural response:

   A: こうこうせいのまえにいます。

   Note that Prof. Yamashita is the subject of this sentence even though he doesn't appear in it. This is completely normal in Japanese.