Some verbs in the present are known as stem-changing verbs or radical-changing verbs. There are three groups of stem-changing verbs:
O -> ue
E -> ie
E -> i
With the first group of stem-changing verbs, the letter o in the stem changes to ue in all forms except the nosotros and vosotros. That is why they are also called 1 2 3 6 verbs, because those are the parts that change and not 4 and 5.
| dormir |
| duermo |
| duermes |
| duerme |
| dormimos |
| dormís |
| duermen |
Notice that the endings are the same for regular -AR, -ER or -IR verbs and it is the stem that changes.
Here is a list of common o -> ue stem-changing verbs:
| almorzar | to eat lunch |
| mostrar | to show |
| colgar | to hang |
| dormir | to sleep |
| probar | to prove, test, sample, taste |
| costar | to cost |
| devolver | to return (an object) |
| resolver | to solve |
| volver | to return (from someplace) |
| encontrar | to find |