は (contrast)
(this one, at least); but as for
Noun (+ particle) + は, contrasting with another
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Besides marking the topic, は sets one thing against another: 肉は食べますが、魚は食べません, "I eat meat, but I don't eat fish".
It often appears twice, once in each half, with が or けど joining them. But even a single は can hint at a contrast: ビールは飲みません, "beer I don't drink (though I might drink other things)".
It replaces を and が. With other particles, it goes after them: 大阪には, 家では, 友達とは.
This is why は is so common in negative sentences: saying "not X" invites the question "then what?", and は answers it.
Examples
肉は食べますが、魚は食べません。
I eat meat, but I don't eat fish.
英語は話せますが、フランス語は話せません。
I can speak English, but not French.
平日は忙しいですが、週末は暇です。
I'm busy on weekdays, but free at weekends.
ビールは飲みません。
Beer, I don't drink.
東京には行きましたが、大阪には行っていません。
I've been to Tokyo, but I haven't been to Osaka.