Is it correct to add a comma before but in the sentence below? An explanation as to why this sentence is correct or incorrect would be appreciated.
If you have a weak stomach, I would not recommend that you watch this film, but if you have no problem seeing blood and guts, then this film will be perfect for you.
The "but" joins two independent clauses. Most manuals of style recommend a comma before such a conjunction. They also recommend a comma after introductory subordinate clauses (i.e., your two "if" clauses).
Yes, the "but" would be connecting two independent clauses. If you are confused due to the subordinate clauses (if statements), you can always move them to the end of the sentence to see if you have two independent clauses to connect:
"I would not recommend that you watch this film if you have a weak stomach, but this film will be perfect for you if you have no problem seeing blood and guts."
I would omit the word "then" in either case as it is unnecessary.
You don't need a comma before the but. But you absolutely need a comma after the film. That's because you put one before the I.
Otherwise you'll find yourself creating a garden-path sentence, butting the "I would" clause rather than the "If you have" one. The reader will be lost.
