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

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.

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