When should I use a comma before but, and when not?
Simple, Compound, and Complex Sentences:
Simple sentences contain a subject and predicate which together form a single independent clause. Compound sentences are built using multiple independent clauses that are connected either by the use of punctuation or a conjunction. Complex sentences contain one independent clause and at least one dependent clause.
Answer and Explanation:
You only place a comma before the conjunction 'but' when it is being used as a coordinating conjunction between two independent clauses in a compound sentence. Ex: 'I want to go on a hike, but I promised I would do the laundry first.' In this sample sentence, 'I want to go on a hike' and 'I promised I would do the laundry first' are both independent clauses which are connected by the conjunction 'but'. In order to show that these clauses are independent and avoid creating a run-on sentence, you need to place a comma before 'but'.
In a complex sentence, however, you do not need to put a comma before 'but'. Ex: 'The cat hissed but then settled down to watch its roommate.' In this sentence, there is only one independent clause: 'The cat hissed'. The conjunction 'but' is used to connect this independent clause to the dependent clause, 'then settled down to watch its roommate'. Since two independent clauses are not being joined into a single sentence, you do not need to place a comma before the conjunction.
Learn more about this topic:
from AP English Literature: Exam PrepChapter 12 / Lesson 8
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