By

"By" is often used to explain how something is done.

You learn by studying.

You lose weight by eating right and getting exercise.

If you buy a dozen it will be cheaper.

It's cheaper by the dozen.

"By" followed by a future point in time means "no later than". It's a limit or deadline.

I need the report on my desk by 5 o'clock.

By The Way

"By the way" is used to introduce a comment or change topics in a conversation.

By the way, have I told you that I'm changing jobs?

"By now" refers to the present situation of an action in progress.

They left the office more than an hour ago so they should be at the airport by now.