робив is imperfective
Perfective forms need a prefix:
- робив - зробив
- біг - пробіг
- писав - написав
- казали - сказали
I can't pinpoint when but I remember ти being used with a -л form. Is there a masculine and feminine form of ти?
Wouldn't it sound better if you wrote робив учора since the в/у is between two other consonants?
There is no strict rule, it's more about how it sounds good for you. "Робив вчора" is not that awful, we just "eat" one "в" in fluent speech. But in case of "робив вчора ввечорі" I would certainly do something, like "робив учора ввечорі" or "робив(в)чора увечорі" :-)