Public transport is free for children up to 15 years and seniors from 65 years of age.
Paper ticket must be stamped with time (validated) when you enter a tram. If you change to a different tram it is still valid, do not stamp it again. It is valid until certain time, regardless changes.
You can buy a ticket inside a tram or bus with your credit card from a contact-less machine. This ticket already has time printed, so no need to punch it with time.
You can buy tickets in a mobile app Lítačka - available at Google Play