Use a comma to separate a series of three or more items: I take a dictionary, a notebook, and some paper to class every day.
Use a comma before words like and, but, or, and so to separate two parts of a sentence that each have a subject and a verb.
She needed some work experience, so she got a part-time job. He did not study at all, but he still got an 87 on the test.
Use a comma after an introductory word or expression, such as However, Therefore,
and In conclusion: However, the high price of electric cars means that most people cannot afford on .