We've already learned that the particle は is used to mark the topic of a sentence. Apart from this usage, は is also used as a contrast marker.
You can use the particle は as a contrast marker by using it multiple times.
| I don't like horses, but I like dogs. | 馬 は 好き じゃ ない が、 犬 は 好き よ。uma wa suki ja nai ga, inu wa suki yo. |
Note that in the sentence above, 馬 and 犬 are subjects of the clauses they're in. But to emphasize the contrast, は is used instead of the subject marker が. We cannot use the particle は after the particle が. The contrast marker は can also not be combined with the object marker を, and therefore also replaces it if we want to contrast two objects.
| (I) did not buy a game. (I) bought a notebook. | ゲーム は 買わなかった。 ノート は 買った。geemu wa kawanakatta. nooto wa katta. |
Note that は's role as contrast marker is consistent with its role as topic marker, as there are multiple topics that are talked about: as for A it is X, as for B it is Y.
This becomes even more apparent in the following sentences, which show that the contrast marker does not need to be used multiple times in a sentence, and that making something the topic is already inherently contrasting it with other things that are not the topic.
| As for Mary, I like her. | メアリー は、 好き だ。mearii wa, suki da. May imply there's someone else I don't like. |
| As for the house, I bought it. | 家 は、 買いました。ie wa, kaimashita. Points attention to the house, in contrast to the previous topic talked about. |
