psa

In a sentence

• Mam psa.

  I have a dog.

  In this sentence, 'psa' is the accusative form of 'pies,' which is used because the dog is the direct object of the verb.

• Idę z psem.

  I am walking with the dog.

  Here, 'psem' is the instrumental case form of 'pies,' used with the preposition 'z' to indicate accompaniment.

• Nie widziałem psa.

  I didn't see the dog.

  In this context, 'psa' is again in the accusative case of 'pies,' here governed by the negative past tense verb.