It determines a link between a part of something to its whole. It is the most frequently used preposition in English.
She is a girl of 18.
Some parts of her body were swelled up.
Most of the people are gone.
The plays of Shakespeare are worth reading.
It is used to refer to something that is on the subject or concerning something or someone. It is a simple way of saying approximately.
I was about to hit a truck.
She cares too much about him.
I have about 30 chocolates in my bag.
This blog is all about computers.
It means that with the object or purpose of something. It highlights a suiting of purpose for something or someone.
I will change it for you
I have something beneficial for you
Apple is good for health
I kept an extra spoon for you.
It means that it is accompanied by something or someone.
I came with Alex
I brought an extra can with me
He is fighting with his cousins
Candy is free with the chocolate.
By refers to something is linked to something or someone. This specific preposition is mostly used in a passive form.
Do you travel by plane?
It was done by me
I wrote this by myself.