do
v.aux.
Used with the infinitive without to in questions,negative statements,and inverted phrases:
用于疑问句、否定句和倒装句跟不带 to的不定式连用,见于问句、否句、倒装句中：
Do you understand?I did not sleep well.Little did we know what was in store for us.
你懂吗?我没睡好.我们不清楚留给我们的是什么
Used as a means of emphasis:
用以强调：
I do want to be sure.Do be still!
我确实想肯定.安静!
（在否定的陈述句和命令句里作为助动词）
He didn't come.
他没来.
（在疑问句里作为助动词）
Did he arrive in time?
他按时到了吗?
（作为助动词表示强调）
He did come after all!
他毕竟还是来了!
（作为助动词代替刚刚用过的或要在答覆时用到的动词）
He likes skating and so does she.
他喜欢滑冰她也喜欢滑冰.