Breakdown of Niczym się nie interesujesz.
nie
no / not
nic
nothing
interesować się
to be interested in (+ Instrumental)
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Questions & Answers about Niczym się nie interesujesz.
Why do we use niczym here instead of nic?
The verb interesować się (to be interested in) always requires its object to be in the Instrumental case. Niczym is the Instrumental form of the pronoun nic (nothing).
Why are there two negative words (niczym and nie) in this sentence?
Polish uses mandatory double negation. If you have a negative pronoun like niczym (with nothing), you must also negate the verb using nie. Unlike English, you cannot mix a positive verb with a negative pronoun.
Could I move się to the end and say Niczym nie interesujesz się?
You shouldn't. The reflexive pronoun się is an unstressed word, and Polish strongly avoids putting it at the very end of a sentence if there are other options. Tucking it between niczym and nie sounds much more natural.
Why does the verb end in -ujesz?
The infinitive of the verb is interesować się. Verbs ending in -ować are regular in the present tense, but they drop the -ować and add -uj- before the personal endings. For the second person singular, this creates interesujesz.