What's the difference between the following two sentences?

  1. He is a boy who can speak English.
  2. He is a boy, who can speak English.
  • Welcome to ELL! Generally, you're expected to provide some more details, like where you found this (a website, a textbook, your own creation) and, ideally, your thoughts on the matter. – Em. Sep 26 '16 at 11:32
  • This probably needs to be decided in context. But basically, when reading the phrase, do you put in a natural pause where the comma is? If not, leave it out. In just the phrase you gave, I think the pause (and comma) is unnecessary. – user3169 Sep 26 '16 at 17:53
  • @user3169 Thank you for your comment. Is there any example whera a comma before "who" or "which" changes the meaning of a sentence significantly? – Harutaka Kawamura Sep 27 '16 at 16:00
  • In general no, but context is everything. The most obvious use is to set off a clause. So "He is a boy who can speak English." vs. "He is a boy, who can speak English, that always gets into trouble." – user3169 Sep 27 '16 at 19:39
  • There are sentences where the presence or absence of a comma before "who" or "which" changes the meaning. Such sentences are quite complex. English learners are encouraged to use short, simple sentences. – J. Doe Oct 2 '16 at 19:23

A comma before "who" is used in parenthetical expressions. For example

My youngest son, who has just turned twelve, is a real genius.

As you can see, the "who" clause here adds some information but can be removed without making the sentence incorrect or incomplete. For clarity, the "who" clause must be separated from the other parts of the sentence; you can either use commas, dashes or parentheses.

Similar example with "which":

I am planning to sail across the Pacific ocean, which is the largest body of water in the world.

Your sentence above is not parenthetical; "who can read English" is an essential part of it. Therefore you should not use a comma.

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