Apostrophes are used to signal two things to a reader:
This is when one thing belongs to another. The apostrophe + ‘s’ shows ownership.
Example 1 – The boy’s shoes were left outside the door.
Example 2 – The dog’s paws were dirty.
If the possessive noun is plural and ends in an ‘s’ already, then an apostrophe is placed at the end of the word, but there is no need for the extra ‘s’:
Example 1- The boys’ (not boys’s) shoes were outside the door.
Example 2 - The dogs’ (not dogs’s) paws were dirty.
With singular nouns that end in an ‘s’, you can either add an apostrophe alone to show possession, or you can add an apostrophe + ‘s’. Both are correct, but you should be consistent.
Example 1 – Mr Jones’s clock has stopped. Or - Mr Jones’ clock has stopped.
Example 2 – The class’s homework was due. Or - The class’ homework was due.
This is when we miss out letters from words to shorten them, forming a 'contraction'.
Example 1 - do not becomes don’t.
Example 2 - could not becomes couldn’t.
Example 3 - cannot becomes can’t.
Example 4 - will not is irregular and becomes won’t.
Its (without an apostrophe) shows a relationship of possession, eg
It's uses an apostrophe to show contraction. It's is short for 'it is' (or sometimes 'it has').