Defining vs Non-Defining Relative Clauses

The difference between defining and non-defining relative clauses is one of the most important grammar distinctions at B2 level. Defining clauses identify which person or thing we mean — they are essential: "The students who passed the exam celebrated" (only those who passed, not all students). Non-defining clauses add extra information about something already identified — they are not essential: "My mother, who is a nurse, works at the hospital" (I only have one mother, so the clause just adds extra detail).

Three rules separate these two types. First, non-defining clauses always have commas; defining clauses never do. Second, you can use that in defining clauses ("The book that I read") but never in non-defining clauses ("The book, which I enjoyed" — not "that"). Third, you can remove a non-defining clause and the sentence still makes complete sense; removing a defining clause changes the meaning. Cambridge B2 exams test this contrast in open-cloze and sentence-transformation tasks, so mastering both types and their punctuation is essential for exam success.