I have to go see the doctor.

我得去看医生。

She made an appointment for her son to see the doctor.

她为儿子约定了看医生的时间。

You don't look well; you ought to see the doctor.

你看起来不太舒服，你应该去看看医生。

When you are ill, see the doctor as soon as possible.

有病要及早治。

Why don't you go to see the doctor?

你怎么不去看医生呢？(表示说话人的建议。)