Hi Christopher. When information is EXTRA, we use a comma. (This is called a non-defining relative clause). When the information is vital to what we're saying, we don't use a comma. (This is called a defining relative clause). Here's an example: My brother who lives in America is 42. (I'm describing 'my brother who lives in America'. All this information is necessary, none of it is 'extra', and I can't take any of it out. I have a brother who lives in Ireland, one who lives in Australia and one who lives in Peru. So 'my brother who lives in America' tells you which one I mean). My brother, who lives in America, is 42. (Here the information is EXTRA. I have one brother. I could simply say 'my brother is 42', but I choose to give you extra information (he lives in America). Therefore I need commas to separate it. I hope that helps.

Thank you very much, Joan! Your answer is extremely helpful! I'm impressed by your kindness! Thank you!! Happy new year! ^^

Hi Christopher, I see Sheena gave you a very good answer to your question. But I also note that she responded to the "who" but did not elaborate on the "which" except to say you should use "which" to refer to the course. So, let me add a bit more infor so you know how to apply a comma before "who" and "which" in any situation: 1. Who refers to people, while "which" refers to things, events, groups, etc. 2. A comma is used before "who" when a () or -- would be necessary for context. Let's use Sheena's e.g: "My brother, who lives in America, is 42". N/B: Without the comma the sentence reads: "My brother who lives in America is 42". If all of this info is vital, then a comma is not needed. However, as Sheena said, if the subject is about the brother, then the other info given would be "EXTRA" and then is offset by a (). e.g. "My brother (who lives in America) is 42". All three examples are correct. In English writing, it is always about the context. 3. A comma is used before "which" to introduce a nonessential/nonrestrictive clause, or what Sheena calls a non-defining clause (they all mean the same thing). e.g. "I am his tutor and also instructor of a course he is taking, which is about personal development". We didn't need to know what the course is about. Maybe we do. But the point is, in this sentence, we didn't. Taking away that nonessential clause didn't change the main info. i.e. you are his tutor, and you teach a course that he is taking. Btw, don't get confused if you see "which" being used to introduce essential clauses. English can be complicated. :) 4. "Which" is also used in a long sentence when you already have a "that" in that sentence and want to add another clause and avoid awkwardness. :) The clause can be essential or nonessential. 5. "Which" is also used in a short sentence when "that" immediately precedes "which." I like this e.g.: "That which doesn't kill you makes you stronger." Hope you found value in what I added. Happy New Year!