Every day you need to say which person or which thing you mean. Imagine you are at work and you say, "Give the report to the woman." Which woman? There are five women in the office. A relative clause solves this in one smooth sentence: "Give the report to the woman who sits near the window." Without relative clauses, you must speak in short, broken pieces: "Give the report to the woman. She sits near the window." That works, but it sounds slow and childish, and listeners can lose the connection between your sentences.

Relative clauses also let you add extra, friendly information without starting a new sentence: "My brother, who lives in Canada, is visiting next week." This is how fluent speakers pack more meaning into less time. If you use the wrong word, like "the person which called you," people will still understand, but the mistake is very visible. Learning the small set of words that start these clauses is a fast way to sound more natural.

A relative clause is a small sentence that sits inside a bigger sentence and describes a noun. It usually starts with a relative pronoun: who, which, that, whose, where, or when. The clause comes right after the noun it describes. In "the man who fixed my car," the clause "who fixed my car" tells you which man.

The first rule is about choosing the pronoun. Use "who" for people: "the doctor who helped me." Use "which" for things and animals: "the phone which broke yesterday." In everyday defining clauses, "that" can replace both: "the doctor that helped me," "the phone that broke." Use "whose" for possession: "the girl whose bag was stolen." Use "where" for places: "the cafe where we met," and "when" for times: "the day when we arrived."

There are two kinds of relative clause, and they do different jobs. A defining clause tells you which one — the information is necessary. "Students who study every day pass the test" means only those students, not all students. There are no commas around a defining clause. A non-defining clause only adds extra information about something we already know. "My mother, who is seventy, still works." You already know who your mother is; the clause just adds a fact. Non-defining clauses always have commas, and you cannot use "that" in them — say "my mother, who is seventy," never "my mother, that is seventy."

One more helpful trick: in defining clauses, when the pronoun is the object of the clause, you can drop it completely. "The film that I watched was great" becomes "The film I watched was great." This works because "I" is already the subject inside the clause. But when the pronoun is the subject, you must keep it: "The film that won the prize was great" cannot lose "that," because the clause would have no subject.

How do you build one? Start with two simple sentences that share a noun: "I met a woman. She speaks five languages." Replace the repeated word ("she") with the right pronoun and attach the clause to the noun: "I met a woman who speaks five languages." Notice that "she" disappears — the pronoun "who" does its job. Keeping both ("a woman who she speaks") is a common error we will look at below.

1. 1. The nurse who checked my blood pressure was kind.

   "Who" starts a defining clause about a person, with no commas.

2. 2. I bought a jacket which keeps me warm.

   "Which" describes a thing in a defining clause.

3. 3. Do you know the man that owns this shop?

   In defining clauses, "that" can replace "who" for people.

4. 4. The soup that you made was delicious.

   "That" is the object here, so you could also drop it.

5. 5. The bus I take every morning was late today.

   The object pronoun is dropped: "the bus (that) I take."

6. 6. She married a man whose family lives in Spain.

   "Whose" shows possession — the family belongs to the man.

7. 7. This is the hotel where we stayed last summer.

   "Where" introduces a clause about a place.

8. 8. I remember the day when we first met.

   "When" introduces a clause about a time.

9. 9. My boss, who travels a lot, is in Tokyo now.

   Commas show a non-defining clause: extra information about a known person.

10. 10. Our new office, which opened in May, is very modern.

    Non-defining clause about a thing uses "which," never "that."

11. 11. People who eat too much sugar often feel tired.

    The defining clause limits the meaning to some people only.

12. 12. Is there a pharmacy that stays open at night?

    Relative clauses work inside questions too.

13. 13. I did not like the movie that we watched on Friday.

    A defining clause in a negative sentence; "that" is the object.

14. 14. The keys which were on the table are gone.

    "Which" is the subject of the clause, so it cannot be dropped.

The most common mistake is choosing "which" for a person. Learners say "The person which called you is outside," but people need "who" or "that." Say "The person who called you is outside." The word "which" is only for things, animals, and ideas. English listeners notice this error immediately because "which" makes the person sound like an object.

The second mistake is keeping the old pronoun inside the clause. Learners write "the woman who she works here" or "the phone that I bought it yesterday." This is wrong because the relative pronoun already fills that position. Say "the woman who works here" and "the phone that I bought yesterday." One job, one word: when "who" or "that" enters, the extra "she" or "it" must leave.

The third mistake is using "that" in a non-defining clause, or forgetting the commas. "My father, that is a chef, cooks every Sunday" is wrong; say "My father, who is a chef, cooks every Sunday." The commas and "who" work together: when the clause is only extra information, use commas and never "that." Missing commas can also change meaning — "my sister who lives in Rome" suggests you have more than one sister.

• Who is for people; which is for things; that can do both, but only in defining clauses.
• No commas means the clause is needed; commas mean it is only extra information.
• Never use "that" after a comma — write ", who" or ", which."
• When "who," "which," or "that" enters the clause, delete the repeated pronoun (he, she, it).
• You can drop the pronoun only when it is the object: "the book (that) I read."
• Whose = possession, where = place, when = time.