中に vs 中 - When to use に. テスト中に私の携帯が鳴ってしまった

Question

テスト中に私の携帯が鳴ってしまった。

Means: My phone accidentally rang during the test.

Can someone explain why the particle に is required after the 中?

Why does: テスト中私の携帯が鳴ってしまった。

Not work here?

Answer 1

Here に is an article used with time. In spoken Japanese, it can be considered obvious and dropped completely. In spoken Japanese, it would be replaced with a pause, or, if writing out spoken Japanese, with a comma to indicate the pause.テスト中、私の携帯が鳴ってしまった。

Answer 2

テスト中に sounds more natural than テスト中 (without に) in this sentence.

This is because the phone ringing is something that happened at a specific point during the exam. に has the effect of putting focus on that specific point of time. テスト中 (without に) sounds a bit incomplete and vague.

Though the difference may not be so obvious as with 間, these are related.

• In this particular sentence is 間 and 間に interchangeable?
• Difference between 間 and 間に