Should I use a comma before "not"?
Commas are often used to set off a contrasting element in a sentence. These contrasting elements often start with "not": e.g. He chose the green, not the red., It happened at night, not during the day.
When not starts a contrasting phrase then you should proceed it with a comma. Not can also be used as a simple adjective. In this case, you wouldn't use a comma before not: e.g. He is not happy., She is not going to come.
Should I use a comma before "yet"?
Yet can be used to start a contrasting element in a sentence, e.g. She was sad, yet relieved.
When yet is used to set off a contrasting element of a sentence then it should have a comma before it, just like with not.
Other examples of contrastive elements that should be preceded with a comma are:
- You're coming, aren't you?
- The statue seemed different, almost alive.
- The politician seemed stupid, possibly even moronic.
This last example shows that sometimes contrasting elements can be disguised with an adverb before them. So sometimes you might have to look at the second word of a contrasting element to decide whether or not it needs to be proceeded with a comma.