Present Participle (Gerund)
The Spanish gerund is formed from the infinitive of a verb by removing the (ar), (er), or (ir) ending and adding (ando) for (ar) verbs, and (iendo) for (er) and (ir) verbs:
In English it is translated as the “-ing” form of the verb (for example, “speaking”), which has lead to the frequent mistake of calling this form the “present participle”.
Verbs of the second and third conjugations for which the stem ends in a vowel, change the ending -iendo to -yendo:
leer ---> leyendo (reading) (example sentences)
caer ---> cayendo (falling) (example sentences)
The gerund for the verb ir (to go) is also irregular:
ir ---> yendo (going) (example sentences)
Third conjugation verbs with stem changes:
Those which have the changes o>ue>u [e.g., dormir, duermo, durmió] use that u which appears in preterit third-persons forms in the gerund:
dormir ---> durmiendo (sleeping) (example sentences)
morir ---> muriendo (dying) (example sentences)
Those which have the changes e>ie>i [e.g., sentir, siento, sintió] use that i which appears in preterit third-persons forms in the gerund:
sentir ---> sintiendo (feeling) (example sentences)
Those which have the changes e>i>i [e.g., pedir, pido, pidió] use that i which appears in preterit third-persons forms in the gerund:
pedir ---> pidiendo (asking for) (example sentences)
Uses. The gerund is primarily used: with estar to form the progressive tenses; with verbs of motion and seguir/continuar; to introduce an adverbial phrase or express “by (do)-ing (something)”.
Progressive tenses: estar + gerund
To form a progressive tense, use the appropriate tense of the verb estar immediately in front of the gerund, e.g.:
No me molestes; estoy trabajando.
Don't bother me; I'm working.
Estábamos almorzando cuando llegaron.
We were eating lunch when they arrived.
Estuvimos haciendo ejercicio por tres horas.
We were exercising three hours [Or: We spent three hours exercising.]
Dudo que estén practicando a esta hora.
I doubt they're practicing at this hour.
Note that any tense can be used to form a progressive, even the preterit as given in the third example above [note that there the action is viewed as having been limited to a three hour period]. However, the present and imperfect tenses the ones most frequently seen.
Caution: As the name indicates, the progressive tenses express an action viewed as being in progress. Do not use the progressive for other purposes, such as for expressing a future action: “We're leaving for Mexico tomorrow” (Salimos para México mañana [NOT Estamos saliendo).