Missing Comma In A List
When creating a list of two or more things, commas should be used to separate each category in the list. There is no formula for this: you must use logic and reason to understand the categories. Some sentences use several verbs, so the commas will separate the items which belong to a different verb. Some items can be separated by adjectives. Frequently, errors of this nature occur between the last two items, so look before the and (see Oxford Comma).
N.B. American English requires the use of a comma before the last and in a list; British English does not. Be sure to follow local protocol, particularly in formal writing.
I cleaned the house and the garage, raked the lawn, and took out the garbage.
The list in this sentence categorises items by verb. Without the comma after and, readers would be looking for another item which was being raked: I…raked the lawn and ___.
Mary bought the shirt in red, purple, and black.
This sentence doesn’t need a comma before the and because the three colours refer to the shirt Mary bought. The final comma should be removed.
For the bouquet of roses, I selected three red ones, three white ones, and two black ones.
Here, the categories are being defined by the numbers and colours of roses. Because two black ones is a different category than three white ones, we need the comma before the and.