You use the expression Dear Sir at the beginning of a formal letter or a business letter when you are writing to a man.
Dear Sir, Enclosed is a copy of my CV for your consideration.
尊敬的阁下,兹附上我的简历一份,请予以考虑。
sirs
People sometimes say sir as a polite way of addressing a man whose name they do not know, or an older man. For example, a shop assistant might address a male customer as sir.
Excuse me sir, but would you mind telling me what sort of car that is?
打扰一下,先生,您能告诉我那是什么类型的汽车吗?
Sir is the title used in front of the name of a knight or baronet.
She introduced me to Sir Tobias and Lady Clarke.
她把我介绍给托拜厄斯爵士和克拉克夫人。