は

は marks the topic, the thing the sentence is about. It goes after a noun, and the rest of the sentence says something about it. As a particle it is read "wa".

The topic need not be the one doing anything. Whatever the sentence comments on can be the topic, and the comment can carry its own subject with が.

は or が

は sets up something already known, and the news comes after it. が marks the noun itself as the news.

A question word is always the news, so it takes が and never は.

• × 誰は来ましたか → 誰が来ましたか

は for contrast

Two は in one sentence set the two items against each other.

A single は can do the same with the other side left unsaid. That is why a negative sentence so often has は where you'd expect を or が.

は with other particles

は takes the place of が and を. After に, で, から and まで it is added on instead: には, では, からは.

• × 本をは読みました → 本は読みました