Learn Spanish (Latin America): Mientras el lobo bebía agua, el oso lo asustó. - While the wolf was drinking water, the bear scared it.

This is a classic background action interrupted by a sudden event. bebía (was drinking) is in the imperfect because it describes an ongoing background action. asustó (scared) is in the preterite because it is a single, completed action that interrupted the drinking.

Is mientras a clue that we should use the imperfect tense?

Yes! Mientras means while, so it naturally sets the scene for an ongoing action. In stories, the action that directly follows mientras is almost always in the imperfect, like mientras el lobo bebía.

What does the lo in lo asustó refer to?

lo is a direct object pronoun meaning it or him. Here, it refers back to el lobo (the wolf). Even though animals can be translated as it in English, Spanish uses lo to replace masculine nouns and la for feminine nouns.

Why is lo placed before asustó instead of after it?

In Spanish, object pronouns like lo usually must go right before a conjugated verb. So, instead of using the English word order (el oso asustó lo, which is incorrect), you must put the pronoun first: el oso lo asustó.