Non-Defining Relative Clauses: Add a Comma, and the Meaning Changes
Have you ever had this experience: two sentences look almost identical, with only a comma's difference, but the meaning changes entirely? This is very common in English, especially with relative clauses. First, feel the contrast in this pair:
- My sister who lives in Tokyo is a teacher. (My sister who lives in Tokyo is a teacher.)
- My sister, who lives in Tokyo, is a teacher. (My sister—who, by the way, lives in Tokyo—is a teacher.)
The first sentence implies that you have more than one sister, so you need "lives in Tokyo" to identify which one you are talking about. The second sentence indicates that you have only one sister, and "lives in Tokyo" is just a piece of extra information. The same words, but with a pair of commas, convey a completely different message. Today, let's clear up how this comma works.
First, Distinguish Between the Two Types of Relative Clauses
A relative clause is when words like who, which, or that are placed after a noun to provide more information about it. It is divided into two types:
- Defining relative clauses: Without it, you wouldn't know which one is being referred to. It provides essential information and does not use commas.
- Non-defining relative clauses: The noun itself is already very clear, and the clause merely provides extra information. It must be enclosed in commas.
Our main focus today is non-defining relative clauses. Their core principle is: "If you remove this piece of information, the sentence still stands and remains perfectly clear."
Look at these two sentences:
- The book that I borrowed last week is overdue. (The book that I borrowed last week is overdue.)
- War and Peace, which I borrowed last week, is overdue. (War and Peace, which I borrowed last week, is overdue.)
In the first sentence, you have many books, and you rely on "borrowed last week" to pinpoint which one it is—this is a defining relative clause. In the second sentence, the book's title is already clearly stated, and "borrowed last week" is just a casual extra detail—this is a non-defining relative clause.
Rule 1: Non-Defining Clauses Must Be Enclosed in Commas
A non-defining clause is like a whispered aside inserted into the middle of a sentence. When it is in the middle of a sentence, it needs commas both before and after it; when it is at the end of a sentence, just add a comma before it.
- My phone, which I bought last year, still works perfectly. (My phone, which I bought last year, still works perfectly.)
- I gave the keys to Mr. Chen, who lives next door. (I gave the keys to Mr. Chen, who lives next door.)
How to remember: If you cover the part between the commas with your hand, and the remaining sentence still makes sense and is complete, then it is indeed a non-defining clause, and the commas are correctly placed.
- My phone still works perfectly. ✓ (Even after covering the extra information, the sentence is still complete.)
Rule 2: Non-Defining Clauses Cannot Use "that"
This is the most common pitfall, so please keep it firmly in mind: non-defining clauses can only use who (for people) and which (for things), and never that.
- Taipei 101, which opened in 2004, is a landmark. ✓
Taipei 101, that opened in 2004, is a landmark.✗
In other words, if you see that right after a comma, you can be almost certain it is incorrect. That is a frequent guest in defining clauses, and it does not get along with commas.
- The restaurant that we tried yesterday was great. ✓ (Defining clause, no comma, using that is perfectly fine.)
- Maru, who we met yesterday, was very kind. ✓ (Non-defining clause, using who.)
Tip: First check if there is a comma. Comma present → use who / which; no comma → who / which / that are all fine.
Rule 3: Relative Pronouns in Non-Defining Clauses Cannot Be Omitted
In defining clauses, when the relative pronoun acts as an object, we often omit it to make the sentence sound more casual:
- The song (that) you played was beautiful. ✓ (that can be omitted.)
Non-defining clauses do not have this freedom. The who or which inside them must always be kept.
- This song, which my dad loves, is from the 80s. ✓
This song, my dad loves, is from the 80s.✗
The reason is simple: a non-defining clause is extra information. Without the relative pronoun, that segment would stick directly to the main clause, making it read like two sentences awkwardly crammed together. Keeping who / which ensures the extra information is neatly framed.
Rule 4: "which" Can Refer to the "Entire Situation Described Before It"
There is also a very useful advanced application for non-defining clauses: which can refer to the entire event mentioned previously, where its referent is the whole preceding sentence.
- She passed the exam, which made everyone happy. (She passed the exam, which made everyone happy.)
- He arrived an hour late, which annoyed his boss. (He arrived an hour late, which annoyed his boss.)
Here, which refers to the fact that "she passed the exam" and "he arrived an hour late." This usage can only be done with which, and it must be preceded by a comma. Master this, and your sentences will naturally flow into "what result this event brought about."
Summarizing the Differences in One Table
When you look at defining and non-defining clauses side-by-side, the differences are clear at a glance:
- Commas: Not used for defining; always used for non-defining.
- Using that: Allowed for defining; not allowed for non-defining.
- Omitting the relative pronoun: Can be omitted in defining clauses when acting as an object; must be kept in non-defining clauses.
- Nature of the information: Defining clauses provide essential information to specify "which one"; non-defining clauses provide extra "by the way" information.
- After removing the clause: Defining clauses leave people confused about who/what is being referred to; non-defining clauses leave the sentence perfectly complete.
Finally, let's wrap up with one last set of sentences to solidify your understanding:
- Students who finish early can leave. (Students who finish early can leave.) — Defining: only the "students who finish early" are allowed to leave.
- My students, who are all very hardworking, finished early. (My students, who are all very hardworking, finished early.) — Non-defining: my students are all very hardworking, and they finished early.
How to Practice in Loopy
The differences created by a comma are easy to forget if you only look at the rules; they need to be encountered repeatedly in real sentences to be truly internalized. Loopy integrates non-defining relative clauses into the dialogues and articles of our graded curriculum, gradually guiding you upward from your current level (A1 to B1), so you can see in natural contexts how commas frame additional information.
- Spaced Repetition review will bring key points like "comma + which / who" and "no that in non-defining clauses" back to your attention right when you are about to forget them, offering a perfectly timed review.
- Listening & Speaking Shadowing lets you hear native speakers pause briefly when they reach a non-defining relative clause, perfectly matching where the comma goes, helping your ears memorize the rhythm.
- When you encounter unfamiliar words, you can look them up on the fly using the built-in dictionary, where the example sentences will also demonstrate the real-world usage of these sentence patterns.
A single comma can change the whole meaning, but you have already grasped the trick to telling them apart. Practice step by step, and you will find yourself reading more smoothly and writing more accurately.