Final answer:
The comma is used incorrectly in the example sentence by linking two independent clauses without a conjunction, creating a comma splice. The correct usage involves separating the clauses with a period, a semicolon, or adding a conjunction such as 'and.'
Step-by-step explanation:
The comma in the sentence 'The Great Hall is open to the public, you must reserve a spot in advance.' is used incorrectly because it improperly connects two independent clauses without the use of a conjunction. This mistake is known as a comma splice. The correct punctuation would involve using a period, a semicolon, or adding a conjunction such as 'and' or 'but' along with the comma to link the clauses properly.
For example:
- The Great Hall is open to the public; you must reserve a spot in advance.
- The Great Hall is open to the public and you must reserve a spot in advance.
Commas should not be placed after subjects, especially when they form a complete thought or sentence on their own. They should be used to separate items in a list, after introductory phrases, or to set off nonessential information in a sentence.