In this dialogue, why is は used in the question, but が in the answer?


>   1. 田中さん **は** 今日来ますか。
>   2. はい、来ますよ。あ、田中さん **が** 来ました。
>

>
>   1. Does Tanaka san come today?
>   2. Yes, he does. Oh, Tanaka san came.
>

I don't understand the use of が in the second sentence.




When you describe or report information which is newly discovered by
perception that doesn't include assumption or judgement, Japanese grammar
requires you to express it as a sentence whose elements are not topicalized,
in short, without adding particles like は.

That kind of sentences are called 現象文 or 中立描写文 (Sentence of neutral
description). The example of あっ、田中さんが来ました is the case of it.

