Definitions with english words, such as where to put comas?

Use a comma to separate the elements in a series (three or more things), including the last two. "He hit the ball, dropped the bat, and ran to first base." You may have learned that the comma before the "and" is unnecessary, which is fine if you're in control of things. However, there are situations in which, if you don't use this comma (especially when the list is complex or lengthy), these last two items in the list will try to glom together (like macaroni and cheese). Using a comma between all the items in a series, including the last two, avoids this problem. This last comma—the one between the word "and" and the preceding word—is often called the serial comma or the Oxford comma. In newspaper writing, incidentally, you will seldom find a serial comma, but that is not necessarily a sign that it should be omitted in academic prose.

Use a semicolon [ ; ]

to help sort out a monster list:

There were citizens from Bangor, Maine; Hartford, Connecticut; Boston, Massachusetts; and Newport, Rhode Island.

OR

We had four professors on our committee: Peter Wursthorn, Professor of Mathematics; Ronald Pepin, Professor of English; Cynthia Greenblatt, Professor of Education; and Nada Light, Professor of Nursing.

to separate closely related independent clauses:

My grandmother seldom goes to bed this early; she's afraid she'll miss out on something.

The semicolon allows the writer to imply a relationship between nicely balanced ideas without actually stating that relationship. (Instead of saying because my grandmother is afraid she'll miss out on something, we have implied the because. Thus the reader is involved in the development of an idea—a clever, subliminal way of engaging the reader's attention.)

Use a comma to separate the elements in a series (three or more things), including the last two. "He hit the ball, dropped the bat, and ran to first base." You may have learned that the comma before the "and" is unnecessary, which is fine if you're in control of things. However, there are situations in which, if you don't use this comma (especially when the list is complex or lengthy), these last two items in the list will try to glom together (like macaroni and cheese). Using a comma between all the items in a series, including the last two, avoids this problem. This last comma—the one between the word "and" and the preceding word—is often called the serial comma or the Oxford comma. In newspaper writing, incidentally, you will seldom find a serial comma, but that is not necessarily a sign that it should be omitted in academic prose.