Does começa take the mark under the c but comecei does not? One exercise wanted it, the other didn't want it.
If the next letter is an "e" or "i" then "c" is used, and not "ç"
That happens to keep the same sound. The letter C followed by O, U or A sounds like K. To keep the same C sound /S, SS/ you use Ç. So we say começar and começo, but comece, for instance.