Every once in a while,I see fear in your eyes

Every once in a while,I see fear in your eyes.

我总是在你眼中看到恐惧。

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Once in a while指“不时地”，once a bluemoon则指“千载难逢的”。
例：The fact is that,I see him once a bluemoon.
事实上，我很难见到他。