Stem-changing Spanish c>z verbs in the present subjunctive)In Spanish some verbs ending in -cer/-cir undergo a stem change in the present subjunctive, in a very similar way they do in the present indicative. They will switch the -c for a -z The difference is that while in the present indicative only the yo form is affected by this stem change, in the present subjunctive all forms keep the stem change.
Have a look at these tables showing both the present indicative and the present subjunctive of convencer (to convince):
él / ella / usted
|nosotros / nosotras||convencemos||convenzamos|
|vosotros / vosotras||convencéis||convenzáis|
|ellos / ellas/ ustedes||convencen||convenzan|