All verbs end in u, tsu, ru, mu, bu, nu, ku, or gu.
With the exception of some special verbs (kuru, suru and certain ru verbs like taberu) when you're using the -masu forms they all change by dropping the 'u' sound and adding an 'i' sound. So you get:
i, chi, ri, mi, bi, ni, ki, or gi.
For example: kau (to buy)
kau -> kaimasu (present/future tense), kaimasen (negative present/future tense) kaimashita (past tense), kaimasen deshita (negative past tense)
there's also kaimashou (let's "---") which can sometimes work as a future tense, but that's a bit advanced.