You can an "apostrophe" (') when something belongs to something else or some one. You can also use it for contactions. Here are two examples: 1) This is John's car. (Possession) To show that the car belongs to John 2) What's (what is) your name. (Contraction) To contract the words "What" and "is" into "What's" Hope this helps

Gaby, part 1 of Luff's answer above gives incorrect examples. 's can show possession, but you wouldn't use it with pronouns (he, she, me, etc). The correct way to indicate "the bag belongs to her" would be to say "the bag is hers", and the way to say "the purse belongs to me" is "the purse is mine". "Mine" is already a possessive pronoun (like the Spanish word "mio/mia") and you don't need to add 's to it

With singular nouns, to signify belonging. Eg, the boy's school, the girl's book, the man's work-place.

Thanks

1. when something is possessive for example: the bag is her's. this is saying the bag belongs to "her" example 2: this purse is mine's this purse belongs to me 2. when omitting the word "is" ex: she's sad she is sad sometimes s' will be used if something is both possessive and plural ex: this is the dogs' food this is the food belonging to the dogs (multiple dogs)