Note: Wa has several usages, but its basic function is to set off a topic (e.g., of conversation) from the rest of the sentence, which talks about the topic. Technically, wa does not indicate case (subject, object, etc.). However, in practical terms, it often (but not always) comes after the subject of the sentence. See also to wa (#7) and -te wa (#47).
1. Indicates that information is being presented about something that is already known or that has been 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.
2. Indicates a topic, which is then identified or explained.
Ashita wa nichiyobi 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, the noun which it follows is no longer being presented as a topic but as the subject of the predicate (see ga, #2, I-2). The switch from topic (wa) to definite subject (ga) lays stress on the latter. For example:
Asatte wa nichiyobi desu ne.
Chigaimasu. Ashita ga nichiyobi desu yo.
The day after tomorrow is Sunday, isn’t it.
You’re wrong there. Tomorrow is Sunday.
3. In the construction N + wa N + ga, wa indicates a topic (the first noun) about which an aspect or quality (the second noun) is explained.
Zō wa hana ga nagai desu.
The elephant its nose is long. / Elephants have long noses.
Takemoto-san wa seikaku ga yasashii desu.
As for Takemoto, her personality is gentle. / Takemoto has a gentle nature.
4. Used to show contrast between two items or ideas, both of which are signified by wa.
Kanji wa muzukashii desu ga, Nihon-go no bunpo wa amari muzu-kashiku nai-n desu.
Kanji are difficult, but Japanese grammar is not very difficult.
Hokkaidō no fuyu wa samui desu ga, Tōkyō wa atatakai desu.
The Hokkaido winter is cold, but [the] Tokyo [winter] is warm. / It’s cold in Hokkaido in the winter, but warm in Tokyo.
Note: In some cases, only one item or idea is explicitly mentioned. For example, in the following sentence, the implication is that the person might go to a cheaper restaurant.
Takai kara, ano resutoran ni wa ikimasen.
Because it’s expensive, I won’t go to that restaurant. / I am not going to that restaurant because it’s too expensive.
Note: In its contrastive function, wa comes after other particles (e.g., ni wa, de wa). Two important exceptions are when it replaces ga and o, as in the next example.
Batā o kaimashita ka.
Māgarin wa kaimashita ga, batā wa kaimasendeshita.
Did you buy some butter? I bought some margarine, but I didn’t buy any butter. / I bought some margarine, but not any butter.
5. In the forms V-te wa iru (first example below) and V –masu base followed by wa and suru (second and third examples), wa indicates emphasis. See also -te wa (#47).
Konpyuta o motte wa imasu ga, mada tsukatte (wa) imasen.
I own a computer [I do own a computer], but I haven’t used it yet.
Ano hito o shitte wa imasu ga, amari hanashita koto wa arimasen.
I know him, but I haven’t spoken to him much.
Ocha wa nomimashita ga, jikan ga nakatta no de shokuji wa shi-masen deshita.
I had some tea, but since there wasn’t much time, I didn’t eat (have a meal).