Hello friends. i have a question , well, it maybe simple for you but :P i really don't understand the diffrent.
For example :

бегать - побежать  (to run) быть - побыть  (to be)
видеть - увидеть  (to see) говорить - сказать  (to speak)
давать - дать  (to give) делать - сделать  (to do)
...and it goes on.

one of them is for the past tense ? what is it?
Thanks o lot.