You use a comma in a sentence primarily for the following purposes:
-
To show a pause in a sentence or separate items in a list: Commas are often used to separate items in a series, making it clear that each item is distinct. For example: "I bought apples, oranges, bananas, and grapes."
-
To denote an omission: Commas can indicate omitted words in a sentence, particularly in elliptical constructions. For example: "I like coffee; my sister, tea."
-
To indicate an abrupt break in thought or structure: Commas can be used to set off transitional phrases or to indicate a shift in the structure of a sentence. For example: "I wanted to go, but—"
-
To mark the end of a sentence: This is incorrect; a comma is not used to mark the end of a sentence. Instead, a period, question mark, or exclamation mark functions to conclude a sentence.
In summary, commas are primarily used to separate items, show pauses, and clarify sentence structure, but they do not mark the end of a sentence.