By — Means, Method, and Agent

Use 'by' for means of transport (by bus), channels of communication (by email), fixed expressions (by mistake, by heart), and the agent in passive sentences.

By — Means, Method, and Agent

The preposition 'by' is one of the most versatile in English, but its core function in this subtopic is clear: it signals the channel, method, or means through which something is done, and identifies the agent who performs an action in passive sentences. Corpus analysis from the British National Corpus shows that by + transport noun structures (by bus, by train, by car) are among the 50 most frequent prepositional phrases in written English, yet learners at B1–B2 regularly substitute 'with' or 'on'. The zero-article rule (by bus, not by the bus) is a second source of systematic error.

By for Transport and Communication

Use 'by' with means of transport and communication — always without an article:

She goes to work by bus. (not 'by the bus')
We travelled to Paris by train.
I'll send the documents by email.
She confirmed the booking by phone.

By in Passive Sentences

'By' introduces the agent — the person or force responsible for the action:

This novel was written by Leo Tolstoy.
This picture was painted by a local artist.

Fixed Expressions with By

I deleted the file by mistake. (unintentional)
He knows the poem by heart. (memorised)
We met by chance / by accident.
She paid by credit card.

Common Mistakes

✗ She sent the invitation with post. → ✓ She sent it by post.
✗ I contacted her with phone. → ✓ I contacted her by phone.
✗ She travelled by the bus. → ✓ She travelled by bus.