This is the 15902nd most frequent Polish word.
teorią
"Theory."
In this sentence, 'teorią' is used as an instrumental form of 'teoria,' referring to a specific theory that someone possesses or expresses.
Nie zgadzam się z jego teorią na ten temat.
I disagree with his theory on this subject.
Here, 'teorii' is the genitive form of 'teoria,' which is used as part of a prepositional phrase indicating a basis of argument or perspective.
Według tej teorii, wszechświat jest nieskończony.
According to this theory, the universe is infinite.
In this example, 'teorii' is again in the instrumental case, showing the means through which a situation is explained.