Comma Before And

If there are only two things in a list, they do not need to be separated by a comma. If there are more than two ideas, the comma should be used as it would in a list. (See Oxford Comma.)

The purpose of a comma is to clarify lists, and conjoin ideas.

The compound predicate, is to clarify lists and conjoin ideas, does not require a comma. The comma should be removed.

The dog is young but well-trained, and good-natured.

The dog is young, well-trained, and good-natured.

The comma in the last sentence is correctly placed as it is being used to clarify the elements in a list.

A comma should also be placed before and if and is at the beginning of an independent clause.

The purpose of a comma is to clarify list, and it also conjoins ideas.

The dog is young, and it’s also well-trained.

It’s cold outside, and I can’t find my coat.