which one is right? 'el convirtió','ellos convirtieron'? or 'el convertió','ellos convertieron'?

Question

which one is right? 'el convirtió','ellos convirtieron'? or 'el convertió','ellos convertieron'? THANK YOU!

Answer 1

Almost all verbs with a o->ue or e->ie stem change have a o->u or e->i stem change in the preterite third person, both plural and singular.
Servir:
sirvé, serviste, sirvió, servimos, sirvieron.
Dormir:
dormí, dormiste, durmió, dormimos, durmieron.

Answer 2

Look, another complete page:

<http://www.wordreference.com/conj/ESverbs.asp'v=convertir>

Answer 3

convertir:

http://www.esfacil.eu/verbos/conjugar/convertir.html