Write three sentences, each using a comma in a different way. Explain why the comma is needed in each sentence. (4 points)
1. I went to the store to buy some groceries, but they were all out of bread.
The comma is needed in this sentence to separate the two independent clauses (I went to the store to buy some groceries and they were all out of bread) connected by the coordinating conjunction "but."
2. The cat, which had been missing for days, finally showed up at our doorstep.
The comma is needed in this sentence to set off the nonessential or nonrestrictive clause "which had been missing for days," providing additional information about the cat. Without the comma, the sentence would imply that there is only one cat and it had been missing for days.
3. My sister, who is a doctor, just opened her own practice.
The comma is needed in this sentence to set off the nonessential or nonrestrictive clause "who is a doctor," providing additional information about my sister. Without the comma, the sentence would imply that there is only one sister, and she just opened her own practice.