you know something/what?
informal 1
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used to emphasize the statement that comes after it你知道吗（用于强调要说的话） -
You know something? I never trusted her.你知道吗？我从未相信过她。
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She lied to me again, but you know what? I really just don't care any more.她又跟我撒谎了。但是你知道吗？我真的一点也不在乎了。
2
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used to get someone's attention你知道吗（用于引起某人注意）