You use an apostrophe to show that a thing or person belongs or relates to someone or something: instead of saying the party of Ben or the weather of yesterday, you can write Ben’s party and yesterday’s weather.
Here are the main guidelines for using apostrophes to show possession:
Singular nouns and most personal names
With a singular noun or most personal names: add an apostrophe plus s:
We met at Ben’s party.
Personal names that end in –s
With personal names that end in -s: add an apostrophe plus s when you would naturally pronounce an extra s if you said the word out loud:
He joined Charles’s army in 1642.
Note that there are some exceptions to this rule, especially in names of places or organizations, for example:
St Thomas’ Hospital
If you aren’t sure about how to spell a name, look it up in an official place such as the organization’s website.
With personal names that end in -s but are not spoken with an extra s: just add an apostrophe after the -s:
The court dismissed Bridges' appeal.
Plural nouns that end in –s
With a plural noun that already ends in -s: add an apostrophe after the s:
The mansion was converted into a girls’ school.
Plural nouns that do not end in -s
With a plural noun that doesn’t end in –s: add an apostrophe plus s:
The children’s father came round to see me.
The only cases in which you do not need an apostrophe to show belonging is in the group of words called possessive pronouns - these are the words his, hers, ours, yours, theirs (meaning ‘belonging to him, her, us, you, or them’) - and with the possessive determiners. These are the words his, hers, its, our, your, their (meaning 'belonging to or associated with him, her, it, us, you, or them').