1. DO作为助动词时的时态、人称和数的变化与它作为本动词相同,有do, does, did三种形式。
Yes, it seems he doesn't really want to have a haircut.
是啊，他好像不是真的想理发。

Then, why does he come here?
那他为什么来这里呢？

Where did he go when he left here?
他离开这儿去了哪了？

2. DO+主语+原形动词构成疑问句。
How long do I have to wait?
我得等多久？

3. DO+not+原形动词构成否定句。
No, I don't want to wait that long.
不行，我不能等那么久。

4. DO(+not)+主语用于附带问句。
You want to have a haircut, don't you?
你想理发，是吗？

5. DO+原形动词用于加强语气句。
Yes, I do want to have a haircut.
是，我真的想理发。

6. Never (Only)+DO+主语+原形动词用于倒装句。
Never did I see such a strange man.
我从未见过这么奇怪的人。