Commas

1.  When a movable modifier opens a sentence, the movable modifier is set off from the sentence by a comma.

Examples:  If I do something he does not like, he will tell me.
In general, Matthew is thoughtful, kind, and open.

2.  When a movable modifier ends a sentence, the movable modifier is sometimes (not always) set off from the sentence by a comma.

Examples:  I first met Maria in basic training two years ago.
We tried to continue our discussion, although the sound of the electrician’s drill was deafening.

3.  When a movable modifier occurs in the middle of a sentence, the movable modifier is sometimes (not always) set off from the sentence by two commas.

The meaning of the sentence governs the decision to use or not use commas.

Examples:  Steve, concentrating intently on his notes, almost tripped over the baby.
The student concentrating intently on his notes almost tripped over the baby.

Note:  In this case, the first example takes commas because we can tell who the subject is without the information.  The second sentence does not take commas because the information in the movable modifier is necessary to identify the student.

4.  Do not separate the subject part of the sentence from the verb with a comma.  Subject to verb is the most direct relationship in English.

Example:  Soon, the tranquility and peace of the mountains replaced the cluttered worries of my mind. [There is no comma after mountains.]

5.  Use commas to set off items in a series of three or more. The comma before the last and or or is optional, but most sources recommend it for clarity.

Example: I am taking math, English, biology, and art.

Do not use commas for linking only two items, unless they are complete sentences.

Examples:  Each dawn the sunlight would creep in my tent and wake me to the sound of Big Creek rushing by. [There is no comma after tent; the and links two verbs: creep and wake.]

Each dawn the sunlight would creep in my tent, and it would wake me to the sound of Big Creek rushing by. [Two complete sentences joined by the coordinate conjunction and.]

6.  Never start a line of text with a comma on the left margin; always attach it to the end of the line, on the right side.