Learn Portuguese (Portugal): Imaginando que a bagagem esteja extraviada, a companhia aérea dá-lhes um vale. - Imagining that the luggage is misplaced, the airline gives them a voucher.

Imaginando que sets up a hypothetical scenario. Whenever you frame a situation as a pure assumption or possibility rather than an established fact, Portuguese requires the subjunctive. We use the present subjunctive esteja because the result in the main clause (dá-lhes) is also in the present tense.

Since European Portuguese often avoids the gerund (-ndo endings), why is imaginando used here instead of a imaginar?

While it is true that European Portuguese usually prefers a + infinitive for ongoing continuous actions (like saying estou a comer instead of estou comendo), the gerund is entirely standard and natural in Portugal when forming adverbial phrases and hypothetical conjunctions like imaginando que, supondo que, or admitindo que.

Why do we use a form of estar (esteja) instead of ser (seja) for misplaced luggage?

We use estar because being misplaced is a temporary condition or the result of an action, not an inherent, permanent characteristic of the luggage. The luggage was not created lost; it just temporarily ended up that way!

Is extraviada the most common word for lost luggage?

Yes, in the context of airports, transport, and postal services, extraviada (from the verb extraviar, meaning to misplace or misdirect) is the standard, official term. You can use perdida (lost) in everyday casual speech, but extraviada is the exact word you will see on official airport forms and signs.