I am a PhD student in Ukrain. I am 30 years old. I am going to Bremen, Germany tommorow. Does one know can i use discount tickets in Bremen for public transportation or not?
There are typically no discount tickets for students in Germany.
As student, you get a 'Semsterticket' together with your matriculation documents after payment of your semester fee. This ticket allows you to use public buses and trains for free within the state, except long range trains like IC/EC/ICE.
Since Bremen is a small state on its own, the range of its Semsterticket ticket is extended to about half the area of the state Niedersachsen, and also includes some IC trains between Bremen and Emden / Norddeich.
For more details about where the ticket is valid, see this link (scroll down for english version)
Of course, you do pay for the semester ticket with the semester fees.
At my university, PhD students are not required to be matriculated. For many PhD students, the benefits like free public transport and other student discounts did not make up the fee, so they do not matriculate. So, if you decided against it, you have no ticket.
If you have a Semsterticket, you need to put your photo in it, and in addition, need to have your ID card with you.