How to Use Commas
Commas keep ideas separate and provide clues about how phrases relate to each other.
Most of the time, commas belong in predictable places, but it is not necessary to use one if it will cause confusion or make a sentence awkward. For example:
Example 1
While I ate the plate grew cold and the food lost its taste.
Do I mean that I ate the plate? No, of course not. I need a comma or two in order to clarify what I mean here:
Example 2
While I ate, the plate grew cold and the food lost its taste.
Do I need another comma?
Example 3
While I ate, the plate grew cold, and the food lost its taste.
The punctuation in both sentences is correct. Choosing to use one or two commas depends on how closely I want to link the cold plate to the tasteless food. Sentence 1 indicates that the food lost its taste because the plate grew cold since there is no pause. Sentence 2 is more like a list, the pause indicates that the cold plate is not necessarily the cause of the tasteless food, yet the phrases have equal emphasis.
Non-essential Phrases
Commas also set apart non-essential phrases. A good test is how important the information is to the sentence. If the point is clear without the phrase, then it needs two commas to set it apart. For example:
Example
While I ate, chewing slowly as I thought of other things, the plate grew cold and the food lost its taste.
Other situations that usually call for a comma
Lists
A, B, and C - if there are three or more elements, use commas and the word "and." For only two elements, use only the word "and."
- There was pizza, salad, fruit, and bread on my plate.
- There was pizza and bread on my plate.
Lists of adjectives
Use a comma for descriptions not joined by a conjunction word.
- There was hot, delicious pizza and warm, crusty bread on my plate.
Conjunctions
Words like "for," "and," "or," "but," "nor," "yet," and "so" need a comma before them when they connect two independent clauses.
- There was pizza and bread on my plate, but I still needed to get salad and fruit.
- Do not use a comma if either phrase is not a complete thought.
- There was pizza and bread on my plate but no salad or fruit.
Clauses
Use a comma before and after, just like this, to set apart extra information supporting the main point or after introductory words or phrases.
- The pizza on my plate, a large slice, was growing cold.
- Darn, the pizza on my plate was growing cold.
Quotations
Use a comma between your own words and a quote.
- As I ate my pizza I heard someone say, "Chew your food slowly," yet I worried my food would grow cold.
Names
Use a comma in front of geographical names and titles in names.
- The pizza place is located in St Paul, Minnesota.
- Ben Chau, PhD is an expert in food science.