There are several different uses for Wa, but its basic function in Japanese, is to set off a topic (usually of a conversation) from the rest of the sentence, which talks about the topic. To be technical, Wa does not indicate case (subject, object, etc.). However, it usually (not always) comes after the sentence subject. Other uses will be indicated in later parts.「To-Wa and Te-Wa 」
「Indication of information of something that known beforehand, or recently identified」
Asoko ni akai hon ga arimasu ne. Are wa kanji no hon desu.
Over there is a red book, right. It's a kanji book. ・See the red book over there? That's a kanji book.
Ano daigaku wa, yotsuya-eki no chikaku ni arimasu.
That university-it's near Yotsuya Station. ・That university is near Yotsuya Station.
「Indication of a topic which is later identified or explained」
Ashita wa nichiyōbi desu.
As for tomorrow, it's Sunday. ・Tomorrow is Sunday.
Kujira wa sakana de wa arimasen.
As for the whale, it is not a fish. ・ The whale is not a fish.
Note: If ga replaces wa in these sentences,「example 2」 the noun which it follows is no longer being presented as a topic but as the subject of the predicate. The switch from topic (wa) to definite subject (ga) lays stress on the latter.
Asatte wa nichiyōbi desu ne.
Chigaimasu. Ashita ga nichiyōbi desu yo.
The day after tomorrow is Sunday, isn't it?
You're wrong there. Tomorrow is Sunday.
(.=^・ェ・^=) こゆうきあいはら (.=^・ェ・^=)