Some English prepositions carry a single clear meaning. By, with, and about are not among them. Each one covers several distinct ideas, and mixing them up changes the meaning of a sentence. This lesson maps out the main uses of all three.
By
Agent in the passive voice
When a sentence uses the passive, by introduces the person or thing that performed the action.
The mechanics of passive voice are covered separately. Here the point is simpler: by marks who or what did it.
Transport and communication
By + noun (without an article) describes how someone travels or sends something.
The pattern is always by + bare noun: by car, by plane, by taxi, by post, by phone. Once you add a determiner, the preposition switches to in or on: on the 7 a.m. bus, in her car.
One fixed exception: walking is on foot, never by foot.
Deadline
By + a point in time means no later than.
Next to
By can mean beside or at the side of, with a sense of close proximity.
Method (by + -ing)
By + verb-ing explains how a result is achieved.
Measurements and amounts
By appears in dimensions and in changes of quantity.
With
Instrument or tool
With introduces the physical object used to do something.
Accompaniment
With links people or things that go together.
Manner
With + abstract noun describes how something is done.
Having (description)
With can describe a feature that someone or something possesses.
About
Topic or subject
About points to what something is connected to as a subject.
Approximately
About before a number means roughly, more or less.
Concerned with
In questions and after certain adjectives, about focuses on the subject of a feeling or concern.
By vs with: who did it vs what was used
These two prepositions overlap when describing actions, and choosing the wrong one changes the meaning.
By names the agent — the person or force responsible.
With names the instrument — the tool or object used.
Both can appear in the same sentence:
A useful test: if you can replace the word with who or what force, use by. If you can replace it with what tool or what object, use with.
Common mistakes
By: transport
By: deadline vs about
By vs with: agent and instrument
About: missing preposition