Learn Polish: Kto niepostrzeżenie podmienił cukier? - Who unnoticeably swapped the sugar?

The base verb relates to changing or exchanging. In this lesson's context, the prefix pod- adds the meaning of a stealthy or sneaky action. While a word like wymienić just means to exchange, podmienić specifically means to swap or substitute something secretly so that no one realizes it.

Niepostrzeżenie is quite a mouthful. What is the root of this adverb?

It is built from the verb postrzec (the perfective form of postrzegać), which means 'to perceive' or 'to notice'. Adding the negative prefix nie- and the adverbial ending creates a word that translates perfectly to 'unnoticeably' or 'imperceptibly'.

If podmienić already means to swap secretly, isn't adding niepostrzeżenie redundant?

Linguistically, yes, it is slightly redundant! Since podmienił already carries the stealthy meaning from the pod- prefix, adding 'unnoticeably' just acts as an intensifier in everyday speech to emphasize how flawlessly the trick was pulled off.

Why doesn't the direct object cukier have a special accusative ending here?

Because cukier is a masculine inanimate noun. For these nouns, the accusative case is identical to the basic nominative case. If the object were a feminine noun like kawa (coffee), you would see an ending change: podmienił kawę.