So I know you can't use は after an interrogative word like 何 or 誰. However, if
you add a か after them, they're no longer interrogative words, right? So I
should be able to say something like:

> 誰かは教室に入ったみたいだな

But for some reason, it sounds wrong. Can you say that or not? I'm not only
talking about words like 何か or 誰か, but also 何も and 誰も or 何でも and 誰でも.
Basically any interrogative + particle structure. Can they take は?
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2020-11-16T20:33:32.907
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particles particle-は topic
は after words like 何か or 誰か
Yes, 誰か can safely take は if there is a reason to do so. As you said, this 誰か
is not for making a question but is a lexicalized noun meaning "someone". It
behaves almost like other ordinary nouns, and you can attach arbitrary
particles as long as it makes sense (誰かを, 誰かに "to someone", 誰かと "with
someone", 誰かから "from someone", 誰かの "someone's"...).

Then what's the difference between the following two sentences?

  1. 誰か **が** 教室に入ったみたいだな。
  2. 誰か **は** 教室に入ったみたいだな。

Both are grammatical sentences, and we need nothing new or special to explain
the difference. が is used when you plainly introduce 誰か into the discourse,
while は is used to make a "contrast". In addition, although less common, you
can use は after introducing 誰か into the discourse. For example:

  * (お前と俺は教室に入っていないが、)誰か **は** 教室に入ったみたいだな。  
Looks like someone (else) entered the room (although you and I didn't).

  * 誰か **が** 学校にいた。その誰か **は** 教室に入ったみたいだな。  
Someone was in this school. And that someone seems to have entered the
classroom.

  * オフィスに誰か **が** います。  
There is someone in the office.

  * オフィスに誰か **は** います。  
There is at least someone in the office.

The same is true with 何か:

  * この扉を開ける何か **が** 必要だ。  
You need something to open this door.

  * この扉を開けるのに鍵は必要ないが、何か **は** 必要だ。  
You don't need a key to open this door, but you still need something (else).

何も/誰も is not even a noun (it's a negative-polarity adverb), and it doesn't
take any particle including は. 何でも/誰でも does not take は in a nonnegative
sentence, but it sometimes takes contrastive-は in a negative sentence. For
example, 何でもは買えない is "We cannot buy everything", 誰でもは買えない is "Not everyone can
buy it".
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