准确答案为：

I have a toothache. I should visit a dentist.
= I have a toothache. I (ought) to (see/visit) a dentist.
我牙痛.我应该去看牙医.

解释：should do sth = ought to do sth 应该/应当做某事

注：/ 表示可以任意选用其一.