"My mom loves dogs but not cats." "My mom loves dogs and cats."
Why or why not should I use a comma in these sentences?
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Neither sentence needs a comma, but for different reasons.
In My mom loves dogs but not cats. the sentence from "but" on is basically a negated direct object of what Mom loves (what Mom doesn't love). Compare with, My Mom loves dogs, but she is scared to death of cats. Now the comma is required because you have joined two independent clauses together. If "she" is absent from the subordinate clause, some accepted styles would leave off the comma, because it would separate the "is" from its subject. The Chicago Manual of Style, however, uses the comma.
In My mom loves dogs and cats. you have formed a two-item list. As such, the word "and" is the only divider needed. In a longer list (more than two), commas become necessary:
The final "," before "and" is optional and called the Oxford comma. To me, it adds clarity and consistency. In some complex lists, the comma before "and" is actually necessary to parse the sentence correctly. If it is necessary some of the time, why not use it all of the time?