There is a change in the vowel and in the pronunciation. In addition the 3rd person plural the consonant doubles. Quite a lot of changes, but you will get the pattern of this, surely.
Example
Personal pronoun
venir (to come)
soutenir (to sustain)
contenir (to contain)
je
viens
soutiens
contiens
tu
viens
soutiens
contiens
il / elle
vient
soutient
contient
nous
venons
soutenons
contenons
vous
venez
soutenez
contenez
ils / elles
viennent
soutiennent
contiennent
Another change in vowels with actually only one verb having this specialty is the following:
u changes to eu in writing and to oe in pronunciation