grammar - Use は or が with ある when the phrase doesn't explicit the place

I'm studying japanese from not that long and a doubt came to my mind studying ある and いる verbs, so sorry in advance if the question isn't clear.

If we take those two phrases as example:

A.　つくえはへやにあります

B.　へやにつくえがあります

For what I've understood, in the phrase A, I want to underline the location of the desk, in this case the room. In the phrase B, instead, I want to underline that in the room there's a desk, so I give more importance to the object, not to the location.

If I would want to translate the phrase "there's a desk" without pointing out the location, I can choose between:

C.　つくえがあります

D.　つくえはあります

indifferently or, if my previous argument is correct, I have to choose C?

Answer

The basic difference is already covered in this question: What's the difference between wa (は) and ga (が)?

The following is a brief summary. As you can see in the above link, each sentence has two different meanings.

• C. つくえがあります。
  ① There is a desk.
  ② (Among the aforementioned options,) The desk is what exists. [exhaustive-listing]


• D. つくえはあります。
  ① The desk (we are talking about) exists. [thematic]
  ② (By contrast / However,) There is a desk. [contrastive]

Regarding Sentence C:

• ① You are bringing a desk into the current conversation. In English, you mention something firstly using 'a' (e.g., "a boy") and then talk about it using 'the' (e.g., "the boy"). In sentence C, が in つくえがあります is similar in purpose to the article 'a' in English.


• ② is a bit tricky, but this is possible when someone asks you "What do you have at home, a desk or a kotatsu?".

Regarding Sentence D:

• ① When you have been already talking about some desk, and someone asks you "Does the desk exist?" or "Do you really have the desk?" then you would answer, "はい、つくえはあります。"


• ② If you stress は when you read this out loud, it would be "contrastive." For example, when someone asks "Do you have a kotatsu at home?", you could answer, "いいえ、こたつはありません。つくえはあります。". In English, you would directly stress the "desk" in this situation.

So when you simply want to say "(Look!) There is a desk.", you have to use が. If you said "つくえはあります" in an inappropriate situation, it would sound very awkward to native speakers.