Learn Spanish (Latin America): Lo increíble es el templo sagrado que habían tallado en la roca. - The incredible thing is the sacred temple that they had carved in the rock.

Habían tallado is the past perfect tense, meaning "they had carved." It is built using the imperfect tense of the helper verb haber (habían) plus the past participle of the main verb (tallado). We use this tense to talk about an action that was completely finished further back in the past.

Can the past participle tallado change its ending to match la roca or el templo?

No. When forming perfect tenses with haber, the past participle always ends in -o, regardless of the gender or number of the nouns in the sentence. Even if the temple were feminine, it would still be habían tallado.

Who does habían refer to if there's no subject like ellos in the sentence?

The -n ending on habían tells us the subject is "they" (or "you all"). Spanish frequently drops pronouns like ellos when talking about an unspecified group of people or builders, similar to the general "they" in English.

Why is Lo used before the adjective increíble here?