Looking up...
near or next to someone or something
He stood by the door.
He was standing next to the door.
The cat slept by the fireplace.
The cat was near the fireplace while sleeping.
Commonly used to indicate proximity in space.
before a certain time
The report is due by Friday.
The report must be submitted before Friday.
We need to finish by 5 PM.
We must complete the task before 5 PM.
Used to specify a deadline or time limit.
as a means of or through
She travels by train.
She uses the train as a means of transportation.
He communicates by email.
He sends messages through email.
Indicates the method or medium used for an action.
according to or in agreement with
By the rules, you must wear a helmet.
According to the rules, you must wear a helmet.
By his own admission, he made a mistake.
He admitted that he made a mistake.
Used to indicate conformity or agreement.
by means of or through the agency of
The door was opened by a key.
A key was used to open the door.
The song was written by a famous composer.
A famous composer created the song.
Used to indicate the agent or instrument involved in an action.
Remember that 'by' can mean both 'next to' and 'before a certain time'. Context usually makes the meaning clear.
When 'by' indicates who or what does something, it often comes after a passive verb (e.g., 'The book was written by her').
In passive sentences, 'by' introduces the agent (the doer of the action).
Old English 'bi', from Proto-Germanic '*bi', meaning 'around' or 'near'. Related to Old Norse 'bei' and Old High German 'bī'.
The preposition 'by' is highly versatile and can indicate proximity, time limits, method, or agency. Its meaning is often clear from context.