What's the difference between the following two sentences?
He is a boy who can speak English.
He is a boy, who can speak English.
Best Answer
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.
This problem has to do with the use of commas, and not so much with the phrase "such that". "Such that" is considered equal to "so that" in meaning, but "so that" is more common and preferred. "Such that" is really formal.
Commas are usually used to separate independent parts of a sentence. Because of the dependence that "such that" and "so that" have on the phrase before it, the comma is not used there.
Now, some people like to put commas where they pause in speaking in order to separate ideas. And this is only correct if it is truly separating two ideas, and not just to add a pause.
Adding a pause in speech to separate ideas would be the only correct reason to put a comma before "such that".
Putting a comma before "such that" and "so that" isn't wrong, but it is unnecessary.
Best Answer
A comma before "who" is used in parenthetical expressions. For example
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":
Your sentence above is not parenthetical; "who can read English" is an essential part of it. Therefore you should not use a comma.