1. We're old friends. we ... each other for over 10 years (know - past form)
2. The children ... that computer game for 3 hours now. It's time for them to stop. (play - past form)
3. It's hard to imagine now but when he was younger my uncle was a professional football player.
-> It's hard to imagine now but when he was younger my uncle ... professionally (play)
4. The concert started before we got to the theater. -> The concert ... (already)
5. Katia began to learn English about 3 years ago. -> Katia ... about 3 years. (learning)