Why is ‘は’ instead of 'が' or 'を' used here?


I saw a sentence:

> これ **は** 、オンラインで聞けますか。

Why is ‘は’ instead of 'が' or 'を' used here? I think the full sentence should
be like the following.

> あなたはこれ **が** / **を** 、オンラインで聞けますか。




First of all, あなた is never used like the impersonal _you_ in English. It
always refers to a concrete person (often someone you don't know the name of).
Even when it's used in an advertisement, it's understood as referring to the
listener as an imaginary yet concrete potential customer. When a sentence
begins with あなたは, it's specifically talking about that person.

Your second sentence, therefore, will be understood as asking about the
ability of the person you are talking to, but I don't think that's what you
mean here. (It's not a "full" version of the first sentence.) You probably
want to ask about the availability of _this_ for online listening. Then
marking これ as a topic is a natural thing to do. You can still say
これをオンラインで聞けますか (without あなたは) without changing the meaning much. The doer of
the listening will most likely be understand as you as the speaker if said in
the same context in which これはオンラインで聞けますか is natural.

