How do you say...."Excuse me, I need to use the restroom, please wait a moment."
i just think it will be great to use this in any situation.

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不好意思，我想去一下洗手间，能等我一会儿吗？

bu hao yi si,  wo xiang qu yi xia xishoujian  neng deng wo yihuier ma?

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不 好  意思, 我  要 去 卫   生   间,   请   稍   等。
bu hao yi si, wo yao qu wei sheng jian, qing shao deng.

^_^ 

我  通    常   这  么  说 ( I usually speak like this ), 
wo tong chang zhe me shuo.

并   且(and), 洗  手   间=   卫   生   间= restroom
bing qie,        xi shou  jian = wei sheng jian

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shi ting you yong de

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对不起，我要使用休息室，请等一会儿。

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抱歉，我想出去方便一下，你能等我一会吗？
bao qian, wo xiang chu qu fang bian yi xia, ni neng deng wo yi hui ma?