Learn Spanish (Spain): Cuando llegó, el crimen ya había ocurrido. - When he arrived, the crime had already happened.

We use the Past Perfect (Pretérito Pluscuamperfecto) to talk about an action that happened before another action in the past. Here, the crime happened first, and then the person arrived. To show this timeline, we use había ocurrido for the earlier event and the simple past (llegó) for the later one.

Why do we use había here instead of another past form of haber?

The Past Perfect tense is always built using the Imperfect tense of the helper verb haber along with a past participle. Since el crimen (the crime) is a singular third person, we use the third-person singular Imperfect form, which is había. You cannot use the simple past form (hubo) to form this specific tense.

Can I put ya (already) between the two verb parts, like había ya ocurrido?

No, in Spanish you can never separate the helper verb (había) and the past participle (ocurrido). Adverbs like ya or siempre must go before or after the entire compound verb, which is why ya is placed right before había here.

How do we know llegó means he arrived and not she arrived?

Out of context, we do not! Llegó is the third-person singular of the simple past, which can mean he, she, or you (formal) arrived. The English translation says he arrived assuming a wider story context, but grammatically it could just as easily be she.