Group 1 verbs

All gerunds are formed by adding -ando to the stem of the infinitive.

example:  parlar; stem is parl + ando = parlando

In group 1, all of the stems are regular.

Group 2 verbs

Gerunds are formed by adding -iendo or -yendo to the stem of the infinitive.

If the stem ends in a consonant, the ending -iendo is used.

If the stem ends in a vowel, the ending -yendo is used.

example:  vender; stem is vend + iendo = vendiendo

example:  creer; stem is cre + yendo = creyendo

In group 2, there is only one irregular verb

poder

pudiendo

Group 3 verbs

Gerunds are formed by adding -iendo or -yendo to the stem of the infinitive.

If the stem ends in a consonant, the ending -iendo is used.

If the stem ends in a vowel, the ending -yendo is used.

example:  abrir; stem is abr + iendo = abriendo

example:  fluir; stem is flu + yendo = fluyendo

In group 3, the following verbs have irregular forms

consentir

consintiendo

Like consentir:

adherir

advertir

arrepentir

asentir

concernir

conferir

controvertir

convertir

deferir

diferir

digerir

divertir

hervir

inferir

ingerir

invertir

mentir

pervertir

preferir

preterir

proferir

referir

rendir

rendirse

requerir

sentir

sugerir

transferir

decir

diciendo

Like decir: 

bendecir

contradecir

desdecir

interdecir

maldecir

predecir

dormir

durmiendo

Like dormir: 

dormirse

morir

morirse

ir

yendo

pedir

pidiendo

Like pedir: 

competir

concebir

derretir

desleir

despedir

embestir

engreir

expedir

gemir

impedir

medir

rendir

rendirse

repetir

revestir

revestirse

servir

vestir

vestirse

freír

friendo

Like freír: 

refreír

reír

reírse

sonreír

sonreírse

venir

viniendo

Like venir: 

circunvenir

convenir

intervenir

prevenir

reconvenir

revenirse