The past perfect tense in Spanish is used to indicate an action that happened before another one in the past. We use it to emphasize that that something happened before the another one.

In English, the past perfect has two parts - often 'had' plus the past simple, eg 'John had gone to London but he got lost.

In Spanish, the pluscuamperfecto del indicativo (past perfect) also has two parts - the imperfect of the verb HABER + past participle.

The Past Perfect in Spanish grammar - another video by 121Spanish

This is how you conjugate the verb HABER in the imperfect tense:

Then you need the past participle. This is how you form the past participle.

Regular past participle forms:

Example: Los ladrones se habían ido cuando ellos llegaron a su casa.

(The burglars had left when they arrived home.)

Irregular past participle forms:

Example: Ellos habían abierto todas las ventanas cuando comenzó a llover.

(They had opened all the windows when it started to rain.)