Hey, it is not a mathematics, it is a natural language. Rule #1: there are always exceptions from any rule and from any exception as well. Once at school I tried to learn all that theoretical staff about conjugations an found that it is too contradictory and there are too many exceptions to take those rules seriously.

Let's see...

звать - зови
брать - бери
драть - дери {a vowel is added to the root}
спать - спи
бдеть - бди {no vowel added}
скакать - скачи
бежать - беги
чесать - чеши {last consonant is changed.}
жать - жми {err... Something is changed.}

Let's see the verbs ended by -сть/ -сти:
класть - клади
вести - веди
пасти - паси
расти - расти
...