By is one of English's most versatile little words, serving many different purposes depending on context.

Most commonly, by indicates who or what performed an action. A book by Mark Twain means Mark Twain wrote it. A painting by Picasso means Picasso created it. When you say “I was helped by my friend,” you're identifying who did the helping.

By also means near or beside something: “Stand by the door” or “The cat sat by the window.” It suggests closeness without touching.

The word indicates methods and means too. You might travel by train, learn by studying, or multiply by three. Here by answers the question “how?”

For time, by sets a deadline: “Finish your homework by dinnertime” means it must be done before or at that time, not after.

In measurements, by shows dimensions: A room that's ten feet by twelve feet tells you its length and width.

Other common uses include passing near something (“drive by the store”), following rules (“play by the rules”), and showing amounts of change (“prices increased by 20%”). Despite being just two letters, by works remarkably hard in English, connecting ideas, showing relationships, and clarifying meaning in dozens of ways.