Learn Spanish (Spain): La alarma estaba sonando en el pasillo oscuro. - The alarm was ringing in the dark hallway.

Estaba sonando is the past continuous tense. We use the imperfect tense of estar (estaba) plus the gerund (sonando) to describe an action that was actively happening or in progress in the past. Using the simple past sonó would mean the alarm rang as a single, completed event, rather than setting the scene of it ringing continuously.

How is the word sonando formed?

It comes from the verb sonar (to ring or to sound). To make this '-ing' form for verbs ending in '-ar', you drop the '-ar' and add -ando. Notice that while sonar has a vowel change in the present tense (like suena), its gerund form is completely regular: sonando.

Why is the adjective oscuro placed after the noun pasillo?

In Spanish, descriptive adjectives like oscuro (dark) almost always come after the noun they describe. This is a good refresher on a basic rule: we say el pasillo oscuro instead of 'el oscuro pasillo'. It also ends in '-o' to match the masculine, singular noun pasillo.

Can I use sonar if I want to say someone was playing music in the hallway?

Not quite. Sonar is used for things that make a noise, ring, or beep, like alarms, phones, and sirens. If you want to talk about a person playing a musical instrument, you must use the verb tocar.