Questions & Answers about Moja matka zaprzyjaźniła się z tą nową sąsiadką.
Why do we need się after zaprzyjaźniła? Does it mean she befriended herself?
Not quite! While się often means "oneself" (reflexive), here it shows a mutual relationship. Just like in your current lesson with verbs like spotykać się (to meet), zaprzyjaźnić się (to become friends) uses się to show that two people are interacting and forming a bond with each other.
Why do all the words at the end change to tą nową sąsiadką?
The preposition z (meaning "with") requires the words that follow it to be in the Instrumental case. This sentence is a perfect example of the feminine singular Instrumental pattern: the pronoun (ta becomes tą), the adjective (nowa becomes nową), and the noun (sąsiadka becomes sąsiadką) all take the -ą ending.
If I want to say "My mother is friends with...", do I still use zaprzyjaźniła?
No, you would use the imperfective verb przyjaźnić się. Zaprzyjaźnić się (used in this sentence) focuses on the completed event of becoming or making friends. To talk about the ongoing state of being friends, you would drop the za- prefix and say Moja matka przyjaźni się z...
Could I put the word się somewhere else in this sentence?