Use the word "man" when referring to a human male, usually someone who is over the age of 18 (in the United States).
A man walked into the room and sat down.
Two men walked into the room and sat down.
Sometimes the possessive form for this noun is confusing. That’s because "man" is an irregular noun. When used in the plural form, the apostrophe goes before the "s." Normally, an apostrophe goes after the "s" in the possessive plural (students‘ books, workers‘ rights).
That looks like a man’s jacket. (singular possessive)
Where’s the men’s room. (plural possessive)
Are these men’s shoes?
A man’s car smashed into the building.
There are a few expressions that use the word "man."
A man’s gotta do what a man’s gotta do. (A man has to take reponsibility or take command of certain situations.)
He’s a man’s man. (Men admire him.)
Who’s the man around the house? (Who makes the decisions and takes care of problems?)
A man’s home is his castle.
Don’t give in to the man. (the man = an authority figure, such as a boss)
You’re the man! (You did something well. Your performance was good.)
Recently, "man" has been used as a verb in the expression "man up." To man up is to act like a man and take care of one’s responsibilities. (I personally think this is a stupid thing to say to someone, but you might hear it.)
The woman told her husband to man up and take care of the problem.
You’re going to have to man up.
He has to man up and talk to his coworker honestly about their differences.
The word "man" is also used in greeting another person or in expressing surprise: