<h2>Right answer: volaban</h2>
In Spanish (Castilian) the imperfect tense (indicative mode) is used to:
1. talk about past habitual actions (adding <em>used to</em> in its different forms)
2. describe what someone was doing when it was interrupted by someone (or something) else. In fact, is more a combination of the past tense and a continuous or repeating aspect. Frequently, this includes a sense of incompletion in the verb, but not always. (Is the equivalent to the Past Progresive Tense in english)
In other words:
It is specially used to describe what used to happen or what was happening.
Now, in this sentence we are talking about birds (pájaros/aves) which is a <em>plural noun</em>.
Then, the imperfect tense of the spanish verb volar (to fly) with the <u>third person in plural</u> ellos (they) is volaban.
Therefore the complete sentence is:
<h2>Los pájaros <u>volaban</u> sobre los árboles.
</h2>
The birds <u>were flying</u> over the trees
