Before is used for something that occurs before a certain time. For example
We should finish this exercise before 4.00
I need to buy a present before James comes
Before starting any business, it’s a good idea to write a business plan
Note that when a preposition is followed by a verb, the verb ends in ing. So for example
Where did you live before coming to America? Correct
Where did you live before to come to America? Incorrect
After is used for something that occurs after a certain time. For example
I’ll see you after work
Camilla won’t be home until after midnight
David got promoted after just two years with the company
As is used when two shorter actions happen together. For example
I dropped my keys as I left my apartment
As is also used when two longer actions happen together. For example
His health improved as he exercised more
As time went by his English got better
As is also used when an action happens during a background situation
I woke up as the sun was rising
She arrived as you were leaving
As is often used to show one thing is the cause of another, like the word "because". For example
I’m going to bed early as I have to get up early tomorrow
As we’re not busy, let’s meet this afternoon for lunch
As soon as is used when something happens immediately after something else. For example
I’ll give you a call as soon as I’m finished
As soon as they go out let’s eat lunch
We’ll send payment as soon as you fax the signed contract
