How would you translate this sentence to Polish: “She was thinking about what her mother told her. “?
It should be: Myślała o tym, o czym powiedziała jej matka.
Where English uses simple “what” to connect to clauses, Polish needs two words. One of them is the reflexive pronoun “to” which needs to be declined. The other one can be either “kto”, “co” or “który” declined as well. Usually you also need to repeat the preposition.
In the exercise which I prepared for you, you will find a chart with declension of “to” sentences which blanks. Powodzenia! 🙂 Na pewno dasz radę!