El Gerundio
The -ando / -iendo form and the progressive
The gerundio is Spanish's equivalent of the English -ing form, but with a much narrower job. It's purely verbal: it describes how or while an action happens, and it powers the progressive with estar. Unlike English, it's never a noun ('smoking is bad') and never an adjective ('running water').
Formación regular
-ar verbs take -ando: hablar → hablando, trabajar → trabajando. -er and -ir verbs take -iendo: comer → comiendo, vivir → viviendo, aprender → aprendiendo. The form is invariable — it never changes for gender or number.
Irregulares
When -iendo would sit between two vowels, the i becomes y: leer → leyendo, oír → oyendo, construir → construyendo, ir → yendo. -ir stem-changing verbs shift the vowel: dormir → durmiendo, sentir → sintiendo, pedir → pidiendo, decir → diciendo, venir → viniendo, poder → pudiendo.
Estar + gerundio (presente progresivo)
Use estar + gerundio for an action happening right now, at this very moment: 'Estoy estudiando' = 'I'm studying (right now)'. Spanish uses this construction much less than English — for routines or general truths, the simple present is preferred: 'Estudio español' (I'm studying Spanish, in general), 'Estoy estudiando español' (I'm studying Spanish right now).
Otras perífrasis con gerundio
seguir / continuar + gerundio → still doing ('Sigo trabajando aquí'). llevar + time + gerundio → have been doing for X ('Llevo dos años estudiando'). ir + gerundio → gradually, little by little ('Voy aprendiendo'). venir + gerundio → has been happening up to now ('Viene diciendo lo mismo desde hace meses').
Cuándo NO usar el gerundio
Never as a noun — use the infinitive instead: 'Fumar es malo' (Smoking is bad), not 'Fumando…'. Never as an adjective — use a relative clause or a different word: 'agua corriente' (running water), 'el niño que llora' (the crying child). And never after prepositions where English uses -ing: 'antes de salir' (before leaving), 'sin decir nada' (without saying anything).
Ejemplos
"Estoy leyendo una novela de Vargas Llosa."
I'm reading a Vargas Llosa novel.
Estar + gerundio — right now.
"Llevo tres meses aprendiendo español."
I've been learning Spanish for three months.
llevar + tiempo + gerundio.
"Los niños están durmiendo."
The kids are sleeping.
Irregular: dormir → durmiendo.
"Nadar es bueno para la salud."
Swimming is good for your health.
Subject = infinitive, NOT gerundio.