영어작문할 때 who나 which를 comma와 함께 써야 하는지 말아야 하는지 헷갈릴 때가 있다.

그 기준은 바로 "단순히 부가적 정보를 제공할 때", 즉 who와 which의 뒤에 나오는 내용이 수식대상을 설명하는 데 있어 반드시 필요한 것이 아닐 때에는 comma를 사용해야 한다는 것입니다.

When to Use Commas with Who and Which

Often there is confusion about when to use commas with who and which. The rule is: If the information provided by the who and which clause (called a relative clause) is just additional information (i.e., it is not required to identify the word it's linked to), then it must be separated from the rest of the sentence with commas.

In other words, if you'd happily put brackets(괄호) around it, then you must at least put commas around it. 

Examples:

• His youngest daughter, who was born on 16 June 1972, swam the channel. 
(The clause who was born on 16 June 1972 is just additional information about His youngest daughter – commas required.)

• James Baker's cat made its own way home after it was accidentally left on the beach at Scarborough. James, who has lived in our village for 10 years, has just won the lottery. 
(The clause who has lived in our village for 10 years is just additional information about James – commas required.)