ga and wa in sentences

hi all, i am new to japan, confused with ga and wa, i hereby seek advice from you guys.
にほんのどうぶつえんにはpandaがいます。why wa is used? can i omit wa?

このきょしつにせんせいはいますか。may i know what does this sentence mean? did i write it wrongly?

your help is greatly appreciated! many many thanks!!

にほんのどうぶつえんにはpandaがいます。
You could drop は but it makes more definite that there are pandas in the Japanese zoos.

このきょしつにせんせいはいますか should be このきょうしつにせんせいはいますか＝Ｉｓ　ｔｈｅｒｅ　ａ　ｔｅａｃｈｅｒ　ｉｎ　ｔｈｉｓ　ｃｌａｓｓ　ｒｏｏｍ？

hi ay, noted your reply with much appreciation! thanks for the correction, i didn't even realize that at all! so for the reply for is there a teacher in the classroom, is it alright the answer to be,"はい、このきょうしつにせんせいはひとりいます。" or "はい、このきょうしつにせんせいはいます。"

can i use ga instead of wa? like this - このきょうしつにせんせいがいますか。

わたしのはなやにはいろいろなはながあります。is いろいろ a な adjective, that's why has to a な behind? the は was there because it wants to emphasize there are many kinds of flowers in the flower shop?

many thanks!!!!

Sorry I can't type in Japanese, I'm on my work computer.

If you're new to Japanese, I would not stress out initially over "wa" and "ga" the different uses can be very nuanced

While it's ok to leave them out of sentences, omitting them from you vocabulary entirely would be highly inadvisable.

However in you're sentence "kono kyousitu ni sensei wa imasu ka" leaving out "wa" would make sense. And may even make more sense. When you're just learning, I would always use "ga" with sentences involving "imasu" and "arimasu" Because "wa" if I were translating the sentence it sounds more like "Well, is there at least a teacher in this classroom?" The "wa" cause the attention to be directed towards the "imasu" more than towards the "sensei"

I know I'm not explaining this well at all! But it's such a weird nuance that is difficult to explain. The best example I can think of to maybe help clarify it would be.
"Kono mise ni hon arimasu ka?" - Are there books in this shop?
"Hon wa arimasen kedo, manga ga takusan arimasu" - We don't have books, but we have a lot of comics.
You say "wa" for the "arimasen" because while you don't have books you have other things. If you had said "Hon ga arimasen" it would sound more like books are the one thing you don't have. The "wa" makes it sound like "Well, I don't have x, ...."

Then "Manga ga takusan arimasu" You're emphasizing the "manga" as the primary point of interest in your sentence. It's a weird nuance, pay attention to when people are using it and it will begin to make more sense

Sorry it's a messy explanation.