A comma and a semicolon are siblings, but they are not twins. In other words, they are NOT interchangeable. In fact, you should only use a semicolon when you have two independent clauses (sentences) without a conjunction (and, but, if, etc.).
You should use a comma when separating words in the same sentence.
Example: Ben ate an apple, a banana, and a peach. CORRECT
You should also use a comma when you have two independent clauses (sentences) connected by a conjunction (and, but, if, etc.).
Example: Sherry went to the store, and she bought celery sticks. CORRECT
If you separate the two independent clauses, you get two sentences: 1) Sherry went to the store. 2) She bought celery sticks.
However, you should never put a comma if you do NOT have two independent clauses.
Example: Sherry went to the store, she bought celery sticks. INCORRECT
If you do not have a conjunction (and, but, if, etc.) and you do not have two independent clauses, do NOT put a comma.
Example: Sherry went to the store and bought celery sticks. CORRECT
This is one sentence because “bought celery sticks” is not an independent clause and therefore is not a sentence on its own. If you were to divide this sentence into two, it would not make sense and it would be incorrect.
Example: 1) Sherry went to the store. 2) Bought celery sticks. INCORRECT
Another instance to use a comma is when there is a natural pause in speech.
Example: She thought to herself, when will I ever go there? CORRECT
Example: She thought to herself when will I ever go there? INCORRECT
Example: Whatever it takes, I will make it. CORRECT
Example: Whatever it takes I will make it. INCORRECT
As for semicolons, they can be used in the place of conjunctions (and, but, if, etc.) when you have two independent clauses (sentences). So, if you are tired of saying “and” and want to change the pace of your sentences, you could insert a semicolon.
Example: Sherry went to the store; she bought celery sticks. CORRECT
As you can see, the semicolon simply replaces the comma and the word “and” in this sentence while still indicating that this sentence is actually two sentences in one. If you replace the semicolon with a comma, it no longer indicates that there are two independent clauses, which is wrong, as shown above.
Bonus semicolon example: Sherry likes tomatoes; however, she does not like potatoes. CORRECT
If you take this sentence and separate the independent clauses, you get this: “Sherry likes tomatoes. However, she does not like potatoes.” Which is also correct.
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