Japanese

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.

Note that in the sentence above, 馬うま and 犬いぬ 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.

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 is it 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.