1. Coordinating Conjunctions:
When but is used as a coordinating conjunction to connect two independent clauses, it is usually preceded by a comma. An independent clause can stand on its own as a complete sentence.
Example:
I wanted to go to the party, but I had a lot of work to finish.
In this example, there are two independent clauses: "I wanted to go to the party" and "I had a lot of work to finish." They are joined by the coordinating conjunction but, and a comma is used before it.
2. Contrasting Ideas:
When but is used to introduce a contrast or opposition between two clauses or ideas, it can be preceded by a comma to emphasize the contrasting nature of the statement.
Example:
The movie was well-made, but it was a bit too long.
In this sentence, the comma before but highlights the contrast between the positive aspect ("well-made") and the negative aspect ("a bit too long") of the movie.
3. Long Introductory Phrases:
If the sentence starts with a long introductory phrase or a series of introductory words, a comma may be used before but to separate the introductory part from the main clause beginning with but.
Example:
After considering all the options available, we decided to go with the second proposal, but it did come with certain risks.
In this sentence, the introductory phrase "After considering all the options available" is followed by a comma before but to clarify the sentence structure and mark the beginning of the main clause.
Remember that the usage of commas before but is influenced by style and context. In some cases, a comma may be optional or unnecessary based on the sentence's flow and overall structure. When in doubt, it's best to consult a style guide or a professional writer to ensure proper punctuation.