Capiscono tutto. – They understand everything.
Finisco il lavoro e arrivo. – I finish work and I come.
However, there is no rule that allows you to determine that verbs are conjugated with the-isc suffix, you just have to memorize them. The most common ones are: capire - to understand, pulire - to clean, finire - to finish, preferire - to prefer, costruire - to build.