1.
~ is added to nouns to form possessives. However, with plural nouns ending in ‘-s’, and sometimes with names ending in ‘-s’, you form the possessive by adding -’.
...the chairman’s son.
...women’s rights.
...a boys’ boarding-school.
...Sir Charles’ car.
2.
~ is the usual spoken form of ‘is’. It is added to the end of the pronoun or noun which is the subject of the verb. For example, ‘he is’ and ‘she is’ can be shortened to ‘he’s’ and ‘she’s’.
3.
~ is the usual spoken form of ‘has’, especially where ‘has’ is an auxiliary verb. It is added to the end of the pronoun or noun which is the subject of the verb. For example, ‘It has gone’ can be shortened to ‘It’s gone’.
4.
~ is sometimes added to numbers, letters, and abbreviations to form plurals, although many people think you should just add ‘-s’.
...new strategies for the 1990’s.
...p’s and q’s.