aber or sondern?
a2sondern only follows a negation and corrects it ('not X but Y'). Everywhere else takes aber. Both are coordinating conjunctions and leave word order alone.
aber
A contrast or qualification, with nothing being replaced.
Das Auto ist alt, aber zuverlässig.The car is old but reliable.
sondern
After nicht / kein / nie: the negated thing is replaced by the right one.
Er kommt nicht heute, sondern morgen.He's coming not today but tomorrow.
The mistake
Ich trinke keinen Kaffee, aber Tee.
Ich trinke keinen Kaffee, sondern Tee.
The tea replaces the negated coffee - which is precisely what „sondern“ is for.
Rule of thumb
Test: is there a negation before it AND is it being replaced? Then sondern - otherwise aber.
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