Contrasting two things
When you want to point out the differences between two things, you usually need to have a sentence structure something like:
"...blah blah, but blah blah..."
When you contrast two things using ‘but’, you usually need to put a comma before the ‘but’, like in this example sentence:
Giganotosaurus may have been larger than T-Rex, but T-Rex is still most people’s favourite dinosaur.
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