I've read that が is the particle used when asking a question, but then I see
sentences like 一番好きな本はなんですか or 一番いい先生はだれですか.

Is there a reason why が is not used in this type of sentence, or is the rule
re: が in a question sentence just a vague guideline?
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2019-01-27T14:32:18.153
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question
grammar particle-が
Can you use the が particle in a sentence asking 何ですか?
I think you have misunderstood the advice you were given. It is not true that
が must be used in questions or that は cannot be. But because interrogative
words like なに, だれ, etc. refer to undefined objects, it does not make sense to
use the topic particle は with them. So if they are the subject of the
sentence, then they _must_ be marked with が rather than は. I believe this is
all they were trying to get at.

For example:

> × だれ **は** 電球を発明したの。  
>  ○ だれ **が** 電球を発明したの。

The question can, however, be rephrased as

> 電球を発明したの **は** だれ

without any problem.
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answer