When you join two complete sentences into a single sentence, you can use commas but follow them with a suitable connecting word: and, or, but, while or yet.
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The group members had to hand in their reports last week, but some were only submitted this week. |
The comma is not required if the subject of the second part of the sentence is omitted or if the conjunction used is and or or.
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The student had to hand in the work by Friday but didn’t make the deadline. |
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The student had to hand in the work by Friday or the work would receive a failing mark. |