[spoken] used when speaking to a man in order to be polite or show respect

先生〔对男子的尊称〕:

'Report back to me in an hour, sergeant.' 'Yes, sir.'

“一小时后向我报告，中士。”“是，长官。”

Can I help you, sir?

我能为您效劳吗，先生？

Sir! You dropped your wallet.

先生！您的钱包掉了。

Dear Sir / Sirs, used at the beginning of a formal letter to a man or to people you do not know

敬启者〔对男子或不知其名者的称呼，用于正式信函的开头〕

Sir, a title used before the first name of a KNIGHT or BARONET

爵士〔冠于爵士或准男爵名字之前的尊称〕:

Sir Paul McCartney

保罗·麦卡特尼爵士

Sir Jasper

贾斯珀爵士

[BrE spoken] used by children at school when speaking to or talking about a male teacher

老师〔中小学生对男教师的直接或间接称呼〕:

Sir, I've forgotten my homework.

老师，我忘记做家庭作业了。

Look out – sir's coming back!

注意 — 老师回来了！

no / yes sir! also no / yes siree!, especially [AmE] old-fashioned spoken used to emphasize a statement or an answer to a question

绝不！/的确！〔用于强调所说的话或对问题的回答〕:

I will not have that man in my home, no sir!

我不要那个人呆在我家里，绝对不行！