Learn Portuguese (Portugal): Ia a sair de casa quando o telefone tocou. - I was just leaving the house when the phone rang.

While estava a sair means "I was leaving," using the imperfect of ir (ia) followed by a and an infinitive adds the nuance of "I was just about to leave" or "I was in the exact moment of leaving." It emphasizes that the action was on the very brink of happening when the interruption occurred.

Is the ia a + infinitive structure common?

Very common, especially in European Portuguese. The structure ir (in the imperfect) + a + infinitive is the standard way to express an interrupted intention in the past. For example, if you say ia a dizer isso, it means "I was just about to say that!"

Why is it de casa and not da casa?

When referring to your own home or home in a general sense, Portuguese omits the definite article, just like we say "leaving home" instead of "leaving the home" in English. You would only use da casa if you were specifying a particular house, like a sair da casa do João (leaving João's house).

How do the verb tenses interact in this sentence?

This is a classic example of setting a narrative scene. The imperfect tense in ia provides the background context (the ongoing or intended action), while the simple past (pretérito perfeito) in tocou acts as the sudden, completed event that interrupts it.