If a phrase is meant to complement or introduce the main information in a sentence, it can be set off by a comma or pair of commas.
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Knowing that some students put things off, the lecturer decided assignments should be submitted a week before the exam. |
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The students in the third group, despite their best efforts, did not submit the assignment on time. |
- Defining relative clauses
In relative clauses do not use commas if the clause defines the antecedent and if omitting it would radically change the meaning of the sentence.
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Students who study here are very intelligent. |
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Students who use the library are likely to be better prepared. |
The above clauses define the type of students referred to.
In defining clauses that do not refer to people use either which or that.
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The research was conducted with materials which were easily obtained. |
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The research was conducted with materials that were easily obtained. |
The above clauses define the type of materials referred to.
- Non-defining relative clauses
Sentences with non-defining relative clauses need commas because the clause adds information to an otherwise complete sentence.
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Students at that university, who were all admitted with academic scholarships, have to work very hard. |
In non-defining clauses, only use which or who.
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The research, which was done over a period of three years, was conducted with easily obtained materials. |
In the following two sentences, which are both correct, the comma makes a difference. In the first, Dr Smith researches only those additives that pose a risk; in the second, the implication is that all food additives pose a risk.
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Dr Smith researches food additives which pose a risk to human health. |
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Dr Smith researches food additives, which pose a risk to human health. |