Here are three rules to follow when using non-defining adjective clauses.
If you follow these simple rules, you will avoid mistakes when you use this type of clause.
1. Always use commas to separate a non-defining adjective clause from the rest of the sentence.
| Do this | Ottawa, which is the capital of Canada, is situated on the border between Ontario and Quebec. | Explanation: Do you see the commas? Non-defining clauses need commas. |
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| Don't do this | Ottawa which is the capital of Canada is situated on the border between Ontario and Quebec. | Explanation: This one doesn't have the necessary commas. |
2. Relative pronouns: never use “that” as a relative pronoun in a non-defining adjective clause.
| Do this | Ottawa, which is the capital of Canada, is perhaps the fourth largest city in the country. | Explanation: The relative pronoun, “which”, is correct. |
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| Don't do this | Ottawa, that is the capital of Canada, is perhaps... | Explanation: The relative pronoun, “that”, is wrong. |
3. Relative pronouns: you can never omit the relative pronoun (unlike defining adjective clauses)
| Do this | I went to their concert, which gave me a good idea of what kind of music they play. | Explanation: This one has a relative pronoun, which is good. |
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| Don't do this | I went to their concert, gave me a good idea of what kind of music they play. | Explanation: This one omits the relative pronoun, which is wrong. |