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> あれは何ですか what’s that?  
>  今何時ですか what time is it now?

I learnt these two sentences from a Japanese learning app. I am confused why
the first sentence uses は whereas the second doesn’t. The English translations
are both in the form “what is...?”
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2019-05-15T22:08:44.083
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question
grammar
When should I use は?
The first point to make is that the first sentence can work without the は
(though it’s slightly informal to drop it) and the second sentence can work
with a は (though it’s more common/natural to not have it).

That said, there is probably a difference in the grammar, which is that あれ is
functioning as the topic in the first sentence, while 今 is functioning more
adverbially in the second sentence (similar to how the “right now” in “what
time is it _right now_ ” works). Perhaps due to this, usage of は in the second
sentence feels like it has some sort of extra nuance, like contrast. That
said, in a very formal context I feel like people would include it regardless
(perhaps as a form of
[hypercorrection](https://en.m.wikipedia.org/wiki/Hypercorrection)?).
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2019-05-15T22:20:50.763
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answer
あれは何ですか means "speaking of that, what is it?" Here, は indicates that the
previous constituent is a topic of our discourse: we're saying we're talking
about _that over there_. Japanese doesn't need to have the topic in every
sentence; if you don't say it, it is assumed you are continuing to talk to
whatever the topic was before, or that it is completely understood what the
topic is. If you just say 何ですか "what is it?", in absence of a previously
established topic in both English and Japanese the most natural response is to
ask for clarification of what the subject was: 何が...？ (or "What is _what_?").
But if someone points out a thingamajig and tells you they want to buy it,
何ですか is perfectly natural - you already know you're talking about the
thingamajig, and re-specifying the topic with あれは is unnecessary.

今何時ですか means "what time is it now?" without any specific topic - it is a very
neutral question, with all the information already present, and nothing
missing. If you just ask 何時ですか "what time is it", either you have already
talked about something (e.g. a friend's wedding, in which case the sentence
will mean "what time is the friend's wedding?") or it will mean "what time is
it now?" simply because it would be stupid to say "what time is it" out of the
blue and expect someone to know you're not talking about anything but now.

You can topicalise pretty much anything (except question words) in Japanese.
Thus, 今は何時ですか "as for now, what time is it?" is perfectly fine. It does gain a
slight contrastive meaning (like if you asked five times in the last ten
minutes, and you're asking yet again - "what time is it _now_?")
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answer