Learn Spanish (Spain): ¿Por qué has escondido las llaves? No las he escondido. - Why have you hidden the keys? I have not hidden them.

In perfect tenses like the Pretérito Perfecto, object pronouns like las must always go right before the conjugated form of haber (here, he). You can never attach a pronoun to the past participle (escondido), and you cannot put any word between haber and the participle.

What exactly does las refer to in the second sentence?

Las translates to "them" and refers back to las llaves (the keys). Because llaves is a feminine plural noun, we must use the matching feminine plural pronoun las to replace it.

Why is the word order No las he instead of Las no he?

When you make a sentence negative, the word no must always go before the object pronoun. The pronoun (las) needs to stick tightly to the conjugated verb (he), so the negative word goes at the very front of the group: No + pronoun + haber + participle.

Why does escondido end in -o if it is talking about the feminine keys (las llaves)?

When a past participle like escondido is used with the helping verb haber to build a tense, its ending never changes. It will always end in -o, regardless of whether the object it refers to is masculine, feminine, singular, or plural.