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.
Creer. CreyendoTo think/believe. Thinking/Believing
Leer. LeyendoTo read. Reading
Oir. Oyendo.To listen. Listening.
Notice how the ending is -yendo, not -iendo.
Another special case with a -yendo ending is verb ir (to go)
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.
Vestir. Vistiendo.To get dressed. Getting dressed.
Seguir. Siguiendo.To continue. Continuing.
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:
Morir. Muriendo.To die. Dying.
Dormir. Durmiendo.To sleep. Sleeping.
Poder. Pudiendo.To be able to. Being able to.
moriendo, dormiendo and podiendo are incorrect!
4. -ir verbs that undergo a vowel change (e > i) and have an -e immediately before the -ir lose the -e from the stem.
Freir. Friendo.To fry. Frying.
Reir. Riendo.To laugh. Laughing.
freiendo and reiendo are incorrect!
See also Forming regular gerunds (-ing form) .
Vestir. Vistiendo.To get dressed. Getting dressed. Leer. LeyendoTo read. Reading Freir. Friendo.To fry. Frying. 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. CreyendoTo think/believe. Thinking/Believing Reir. Riendo.To laugh. Laughing. 
