The construction estar + gerundio is used a lot in Spanish. It is very similar to Continuous Tenses in English.
With perfect tenses it expresses duration. Compare:
– ¿Qué tal la mañana? = How is your morning?
= Bien, he desayunado, he estudiado un poco y he ido al gimnasio = Good, I’ve had breakfast, I’ve studied a little and I’ve been to the gym– ¿Qué tal la mañana?
– He estado estudiando = I’ve been studying
If we talk about a long interval of time, it is much more natural to use estar + gerundio, almost mandatory:
CORRECTO: He estado tres horas estudiando
POCO NATURAL: He *estudiado tres horas
The same occurs in Perfecto Simple and Pluscuamperfecto:
Estuve tres horas estudiando = I was studying for three hours
Había estado tres horas estudiando = I had been studying for three hours
With the imperfect tenses (Presente, Futuro and Imperfecto) estar + gerundio expresses actions that are developing. Compare:
CORRECTO: Leo novelas = I read novels (in general)
INCORRECTO: *Estoy leyendo novelasCORRECTO: Estoy leyendo una novela = I’m reading a novel (now)
INCORRECTO: *Leo una novelaCORRECTO: Mañana cenaré con Juan = Tomorrow I’m having dinner with Juan
CORRECTO: Mañana a esta hora estaré cenando = Tomorrow at this time I’ll be having dinner
INCORRECTO: *A esta hora cenaré con Juan
Unlike English, estar + gerundio never expresses future actions.