sum

sum
es
est

sumus
estis
sunt

Jens mentioned a rule for the thematic vowel: -o- before voiced
sound, nothing before unvoiced. This verb follows the rule: -o-
before voiced sound, -e- before unvoiced.
But remember the rules of sandhi in Latin:
'Dulce et decorum est pro patria mori'
is pronounced
'Dulce-t decoru~-st pro patria mori'

which changes the paradigm into

sum
's
'st

sumus
'stis
sunt


Torsten