By

Preposition

near or next to.
The mailbox is by the bus stop.
at some time before (the given time).
Be back by ten o'clock!
if something is done by somebody, it is done through their action
The matter was decided by the chairman.

The boat was swamped by the water.

He was protected by his body armour.
if something is created by somebody, they are the creator, author, etc.
There are many well-known plays by William Shakespeare.
shows how someone does something
I avoided the guards by moving only when they weren't looking.
using the rules or logic of.
I sorted the items by category.
with a change of. In this case, by shows how much something has changed.
Our stock is up by ten percent.
shows how slowly, or how quickly, someone does something (used in the form "X by X" or "by Xs")
We went through the book page by page.

We crawled forward by inches.
When Template:R the width and depth of something, by is used between the lengths of the two sides.
The window is 45 cm by 60 cm.
If you multiply $x$ by $y$ , you multiply $x$ × $y$ .
Five multiplied by seven is 35.
If you divide $x$ by $y$ , you find how many $y$ s are in $x$ .
35 divided by seven is five.