We use apostrophes (’) for three main reasons.
It is used in writing a contraction to show the place of the omitted letters.
can’t (= cannot)
it’s (= it is)
I’d (= I would/had)
who’s (= who is/has)
We use apostrophes in writing most possessives.
the girl’s father
three miles’ walk
two weeks’ work
my parents’ wedding
Possessive determiners and pronouns do not have apostrophes.
This is yours. (NOT …your’s.)
Whose is that coat? (NOT Who’s …)
Apostrophes are used in the plurals of letters, and often of numbers and abbreviations.
Mind your p’s and q’s.
It was in the early 1960’s.