What is exactly (( pysähtyy))?

Stop = pysähtyä (infinitive! to stop)

Stops = pysähtyy (3rd peron singluar presens)

You can say to people that stop = pysähdy (imperative singular! stop!)

You can say, forexample: that car stops = tuo auto pysähtyy (3rd peron singluar presens)

it helps learners more if you tell them, what kind of forms you use....