The word 'but' is adversative conjunction which are intended to link one-sentence clauses denoting an opposition to what was said earlier, as well as other synonymous conjunctions: however, however, however, however.
Therefore, as with other conjunctions in the Portuguese language, 'but' must be followed by a comma when it is interspersed within a sentence, see the examples below:
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I went to the park, but you didn't show up.
Maria is pregnant but continues to smoke.
The firefighters did their best but were unable to save the house from the fire.
'But' can also function as an adverb when used to reiterate what has already been said, in the sense of emphasizing something. In this case, the use of a comma is also mandatory, see:
The movie made me sad, but very sad.
Although, 'but' it can also assume other grammatical classes that do not have a mandatory comma, as is the case when used as a noun when referring to a defect, as well as if not:
I didn't find one but in your article, congratulations
Nor but not even half but, go now to the bath.
Doctors did everything possible, but the patient did not survive.
He is a good student, but such a good student that he always gets top marks on his tests.
Neither but nor half but, do what I say now!