In Spanish the gerund (the -ing verb form in English) is formed by taking the stem of the verb (e.g hablar → habl-) and adding one of these endings:
-ando (for -ar verbs)
-iendo (for -er/-ir verbs)
However there are some verbs that have irregular gerunds:
1. Verbs ending in -er and -ir with a double vowel in the stem.
Notice how the ending is -yendo, not -iendo.
Another special case with a -yendo ending is verb ir (to go)
Ir → yendo
To go → going
2. Some verbs with an e > i or e > ie stem change in El Presente swap the -e in the infinitive for an -i, however the ending remain regular:
Mentir. Mintiendo.
To lie. Lying.
Preferir. Prefiriendo.
To prefer. Preferring.
3. Some verbs with an o > ue stem change in El Presente swap the -o of the infinitive for an -u, however the ending remain regular:
4. -ir verbs that undergo a vowel change (e > i) and have an -e immediately before the -ir lose the -e from the stem.
See also Forming regular gerunds (-ing form) .
Examples and resources
Preferir. Prefiriendo.
To prefer. Preferring.
Poder. Pudiendo.
To be able to. Being able to.
Mentir. Mintiendo.
To lie. Lying.
Dormir. Durmiendo.
To sleep. Sleeping.
Oir. Oyendo.
To listen. Listening.
Seguir. Siguiendo.
To continue. Continuing.
Morir. Muriendo.
To die. Dying.
Creer. Creyendo
To think/believe. Thinking/Believing
Reir. Riendo.
To laugh. Laughing.
Vestir. Vistiendo.
To get dressed. Getting dressed.
Freir. Friendo.
To fry. Frying.
