Beside, at the side of, next to, close to.
Examples:

• The house is by a river.
• David lives by a train station.
• I would love to live in a house by the sea.

How you travel somewhere.
Examples:

• David went to Manchester by train.
• I go to work by car.
• My parents often go to France by boat.

How you communicate with someone.
Examples:

• I spoke to her by telephone.
• I will send you the invoice by post.
• Please confirm the order by email.

How you pay for something.
Examples:

• We paid for the computer by cheque.
• Can I pay by credit card?

In the passive voice, indicates WHO is doing the action.
Examples:

• The kitchen is being cleaned by David.
• The book was written by Sarah}
• The concert was organised by our school.

Means to do something alone.
Examples:

• I enjoy reading by myself.
• Sarah is studying by herself.
• Let’s do something by ourselves.

How to do something.
Examples:

• You can turn on the radio by pressing that button.

Not later than; before or at a particular time.
Examples:

• Guests must vacate their hotel rooms by 11 am.
• Please send us the payment by tomorrow.
• Students must enrol by the end of June.