Learn Spanish (Latin America): El ladrón corría cuando la policía lo atrapó. - The thief was running when the police caught him.

This is a classic interruption! We use the imperfect corría (was running) to set the scene with an ongoing background action. We use the preterite atrapó (caught) for the sudden, completed event that interrupted the running.

Does the word cuando mean the next verb has to be in the preterite?

Not always, but it is a very strong hint! In stories, cuando (when) frequently introduces a sudden action that interrupts what was already happening. Because of this, the verb immediately following it is usually in the preterite.

Why does lo go right before atrapó?

lo is a direct object pronoun meaning him (referring back to the thief). In Spanish, object pronouns generally must go directly before a conjugated verb. You cannot attach it to the end of a normal conjugated verb like atrapó.

Why does the sentence say la policía atrapó (singular) when the word police is usually plural in English?

In Spanish, la policía usually refers to the police force as a single institution, so it acts as a singular feminine noun. If you want to talk about multiple individual police officers, you would use the plural phrase los policías.