When do I use "What" and when do I use "Which"?

What = open choice (unlimited or unknown options). Which = choice within a limited, known set. "What is your favorite color?" (unlimited). "Which do you prefer, red or blue?" (limited to 2).

What is the most common mistake with "What" and "Which"?

The most common error: using "which" for open options ("Which is the best city?" should be "What") or "what" for a defined set. In informal contexts, native speakers often use both interchangeably, the distinction matters most in formal written English.

How do I remember the difference between "What" and "Which"?

Set test: do the possible options form a limited, already-known group? Yes = "which" (closed set); no = "what" (open set). Signal for "which": when the group of options has already been mentioned ("here are three candidates, which one?").

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Which one to use?Intermediatepronounpronoun

WhatvsWhich

'What' asks from an unlimited set of options; 'which' asks when there is a limited, defined set of options to choose from.

Side-by-side comparison

What

pronoun

"What" is used when the number of options is open or undefined. You are asking about something without limiting the universe of possible answers.

What would you like to eat tonight? I can cook pasta or order something in.
What time does the library close on Fridays? I need to return a book today.
What did she say when you told her about the surprise party this weekend?

Which

pronoun

"Which" is used when the number of options is limited and defined. You are choosing within a specific and known set.

We have chocolate and vanilla ice cream. Which one do you prefer?
Which of these two films do you want to watch first, the comedy or the thriller?
Which seat would you like on the flight, the window or the aisle?

Common mistake

•The most common error: using "which" for open options ("Which is the best city?" should be "What") or "what" for a defined set.
•In informal contexts, native speakers often use both interchangeably, the distinction matters most in formal written English.

Trick to remember it

•Set test: do the possible options form a limited, already-known group? Yes = "which" (closed set); no = "what" (open set).
•Signal for "which": when the group of options has already been mentioned ("here are three candidates, which one?").

Quick rule

•What = open choice (unlimited or unknown options).
•Which = choice within a limited, known set.
•"What is your favorite color?" (unlimited).
•"Which do you prefer, red or blue?" (limited to 2).

Did you know?

WHAT asks for an answer from an open set ("What is your favorite color?"), while WHICH asks you to choose from a limited, known set ("Which do you prefer, red or blue?"). Logicians call this open versus closed reference. The line can blur: "which" is natural whenever you can see or picture the options, even if there are many, as in "Which of these books did you read?"