Learn Hardcore Portuguese (Brazil): Quando a recepção ficou vazia, a recepcionista voltou a sorrir. - When the front desk became empty, the receptionist smiled again.

Breakdown of Quando a recepção ficou vazia, a recepcionista voltou a sorrir.

Why is it ficou and not estava?

Because ficou emphasizes a change of state: the reception area became empty.

So in this sentence, the idea is that first there were people there, then the place emptied out. That change is why ficou is a natural choice.

Why is it vazia and not vazio?

Because vazia agrees with a recepção, which is a feminine singular noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

So recepção is feminine, and the adjective must match: vazia.

What does voltou a sorrir mean exactly?

Voltar a + infinitive means to do something again or to resume doing something.

So:

This structure is very common in Portuguese:

In your sentence, it suggests the receptionist had stopped smiling before, and then smiled again once the reception became empty.