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Put commas before 'which, 'when' and 'who'? But How?

3 min readKarin Littlewood

Looking for help with those confusing comma rules before "which," "when," and "who"?

Many people get stuck when they need to put commas in their sentences. We learned these basic comma rules back in school and forgot them quickly. These three words often make us pause and think about comma placement, especially when we are dealing with different types of sentences.

It is time to learn the simple rules that will make your writing clear. Below is the test that covers all three, followed by examples for each word.

The three rules, one at a time

Ask yourself whether the information is extra or needed. That single question settles almost every case.

  1. 01

    Commas before "which"

    Think of "which" phrases as little side notes you whisper to a friend. If you can remove the "which" part and the sentence still makes sense, use commas. For example: "My phone, which I bought last year, keeps freezing up lately." And another: "The cake, which had chocolate frosting, was already half eaten."

  2. 02

    Commas before "when"

    Use a comma before "when" if you are joining two complete thoughts together, as in "I was watching TV, when the power suddenly went out." Leave the comma out if "when" is needed to complete your thought: "I always get hungry when I smell fresh cookies baking."

  3. 03

    Commas before "who"

    Put commas around "who" phrases when you are sharing extra details about someone: "My brother, who lives in London, is visiting us next month." You could remove "who lives in London" and still have a complete thought. When "who" is crucial to your meaning, skip the commas: "Students who finish early can go out for an extra recess."

Quick checklist

WordComma rule
whichGets commas when it is adding extra details.
whenNeeds a comma if it is joining two complete thoughts.
whoWants commas when it is giving bonus information about someone.

Common mistakes to avoid

Do not sprinkle commas to be safe
People often put commas everywhere just in case. Watch out for adding one when the information is necessary to your meaning.
Not every "which," "when" or "who" takes a comma
There is no automatic rule. Double-check by asking: can I remove this information without changing the meaning?
Let a checker catch what you miss
If you are unsure, a grammar checker can flag common punctuation mistakes and suggest a cleaner sentence structure.

Practice examples

Ready to test your new comma skills on some real sentences? Try fixing these with the rules above.

  1. "The car which broke down yesterday needs to go to the shop."
  2. "Sarah who works at the bank helped me with my loan application."
  3. "I was sleeping when my phone started ringing at midnight suddenly."

And a few more to finish on:

  1. My sister who works as a nurse loves her new job. (Fix this.)
  2. The movie which we saw yesterday was three hours long. (Where do the commas go?)
  3. I was reading a book when someone knocked on my door. (Try this one.)

Wrapping up

Now you know the simple tricks for using commas with "which," "when," and "who." Think of commas as small pauses that help your reader follow your message. Remember the easy test: if you can remove the information, use the commas. Practise on everyday sentences until the rules become automatic.

Check your punctuation before you send it

Paste a paragraph into the grammar checker and see the comma placements you missed.

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