Defining vs Non-Defining Relative Clauses

A defining relative clause identifies which person or thing you mean — without it, the sentence is unclear: "The woman who called is my boss" (which woman? the one who called). A non-defining clause adds extra, non-essential information about something already clear: "My boss, who is from Spain, speaks three languages" (there is only one boss, so the clause is just extra detail). The key signals are punctuation and "that": defining clauses have no commas and allow "that"; non-defining clauses have commas and never use "that." Removing a defining clause changes the meaning; removing a non-defining clause does not.

This is a strict grammar rule in English. "That" can only introduce defining relative clauses — those that identify which person or thing: "The car that I bought." In non-defining clauses (with commas), you must use "who" for people or "which" for things: "My car, which is red, is parked outside" — not "My car, that is red." The reason is historical: "that" has always been used to restrict meaning (saying "which one"), while "who" and "which" can add extra information. In exams, using "that" in a non-defining clause is always marked as wrong.

Commas change a defining clause into a non-defining one, which can completely alter the meaning. Compare: "The employees who speak French will be sent to Paris" (defining, no commas — only those who speak French). "The employees, who speak French, will be sent to Paris" (non-defining, with commas — all employees speak French, and all will go). In the first sentence, speaking French is a condition; in the second, it is just extra information. When writing relative clauses, think carefully: is the information essential to understanding who or what you mean? If yes, no commas. If it is just extra detail, add commas.