Estar + Gerundio in der Vergangenheit

At A1 you used estoy comiendo, "I'm eating". In the past the same construction splits in two, because estar can go into the imperfect or the preterite - and English "I was eating" has to cover both.

The difference in one picture

Estaba lloviendo puts you inside the rain, without saying when it stopped. Estuvo lloviendo dos horas looks at the whole stretch of rain from outside, finished and measured. That is why estuve likes a duration: Estuvimos hablando hasta las tres - We were talking till three. English "we were talking till three" uses the same form for both ideas, so the choice has to come from what you want to say, not from the English.

When not to use the progressive

Spanish uses the progressive much less than English:

• States: verbs like ser, tener, saber, querer, gustar, necesitar do not normally go into it. *Estoy teniendo hambre and *Me estoy llamando Ana are not Spanish; use Tengo hambre, Me llamo Ana.
• The future: "I'm going to Seville tomorrow" is Mañana voy a Sevilla, not *Mañana estoy yendo.
• Ordinary past backgrounds: the simple imperfect is often enough: Cuando llamaste, cenaba. The estaba version stresses that you were right in the middle of it.

The gerund is not an English "-ing" word

The Spanish gerund is a verb form, not a noun. "Reading is my hobby" is Leer es mi hobby, with the infinitive, never *Leyendo es. "No smoking" is No fumar. It does not describe a noun either: "a girl talking on the phone" is una chica que habla por teléfono, and "a box containing books" is una caja que contiene libros. Only a few gerunds work like adjectives, above all hirviendo and ardiendo: agua hirviendo (boiling water), una casa ardiendo (a burning house).

Other helpers with the gerund

Seguir - still / keep: Sigo esperando. Llevar + time - have been doing: Llevo dos horas esperando. Ir - gradual progress: Va mejorando poco a poco. - He's slowly getting better. Acabar - end up: Acabó pidiendo perdón. - He ended up apologising.

Where English speakers slip

• Using the progressive wherever English has "-ing", including for states and future plans.
• Using estaba + gerund for a finished, measured activity: Estuve trabajando ocho horas, not *Estaba trabajando ocho horas when you mean the whole shift.
• Gerunds as nouns or adjectives (*Nadando es sano): use the infinitive or a que clause.