Learn Spanish (Latin America): No los dejó descansar.

When you have a conjugated verb followed by an infinitive, you generally have two choices: put the pronoun before the conjugated verb (No los dejó descansar) or attach it to the infinitive (No dejó descansarlos). However, with causative verbs like dejar (to let) and hacer (to make), placing the pronoun before the conjugated verb is by far the most natural-sounding choice for native speakers.

Why do we use los here instead of les?

In this sentence, the people being allowed to rest are the direct object of the verb dejar. Because they are receiving the action of "letting" directly, we use the direct object pronoun los (them). If the second verb had its own direct object (for example, "He let them read the book"), the people would become the indirect object, and you would use les instead.

Why is descansar in the infinitive form instead of being conjugated?

When using dejar (to let/allow) or hacer (to make/force) to say that someone causes another person to do something, the second verb always stays in its infinitive (unconjugated) form. It is a very straightforward structure: conjugated causative verb + infinitive.

How do we know the sentence means "He didn't let them" if there is no subject word like él?