as quiet as a mouse 悄无声息；一声不响；非常宁静

【例句】
She was as quiet as a mouse that nobody noticed her presence.
她一声不响，以至于没人意识到她也在场。

I'm wondering why Tom was just sitting there as quiet as a mouse.
我搞不懂汤姆为什么不声不响地做在那里。