when you've never done something before.

当你从来没有做过这些事情之前。

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It's something that I wouldn't have done before.

我以前从来不会这么做的。

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because you've already done it or you don't need to do something because you've already done it before.

因为你之前已经做过，或者你觉得因为之前做过就没有必要再做了。

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