sie

In a sentence

• Ona się uczy matematyki.

  She is learning mathematics.

  Here, 'się' is a reflexive pronoun, indicating that the subject (she) is performing the action upon herself (learning).

• Dziecko się obudziło.

  The child woke up.

  In this sentence, 'się' is used as a reflexive particle that makes the verb indicate a state affecting the subject itself.

• Nie martw się, wszystko będzie dobrze.

  Don't worry, everything will be fine.

  'Się' is part of the reflexive verb 'martwić się,' used to construct the imperative mood of not worrying.