は and が with 何 & こそあど


One of my textbooks said that you cannot use the particle は with 何 and
undefined words of the こそあど.

Yet I'm sure I've heard これは何ですか。 many times before, and これが何ですか。 sounds
unnatural to me.

What is the correct usage?




The key word is "undefined". は is a topic marker, and you can't switch topics
to something you don't know about yet.

In 「これは何ですか？」, は is acceptable because これ is a **specific, definite thing**
(at least in context) and 何 is about asking for more information about that
thing.

In「何が好きですか？」. the 何 isn't "definite" to the speaker because it is a
"placeholder for anything" 何. Of course, 好き puts an additional constraint on
it, and if you re-arrange the sentence, you can say 「好きなものは何ですか」 because 好きなもの
is definite enough to switch topics to with は.

**On a side note** , が is the subject marker. Every complete phrase must have
a subject, so you might wonder why sometimes there isn't a が. Sometimes it is
because the subject is implicit... but sometimes it is because the subject is
the same as the topic.

For example, in 「私はアメリカ人です」, 私 is both the topic and the subject. That is, the
actual meaning is (note: this is not correct Japanese for basic subject-
marking use of が!) 「私はがアメリカ人です」. But Japanese has a forced grammatical
convention where the は and が particles collapse to just は when the same noun
is being marked as both subject and topic (except in certain other special
usages of が). In other words, が MUST be abbreviated out in these cases. This
is consistent with the view presented on p.118 of "A Dictionary Of Basic
Japanese Grammar".

