A direct object is a noun following the verb that answers the questions what? or whom?
There are two places where direct object pronouns can be placed.
Before a conjugated verb.
Attached to the end of the verb, ONLY IF the verb is not conjugated, such as infinitives or gerunds or if the verb is an affirmative informal command.
For example: Son muy buenos hijos. No los cambiaría.
They are good kids. I wouldn’t change them.
Direct object pronouns in Spanish agree in number and gender with the nouns they replace. In the example, hijos is a masculine, plural noun so the masculine plural direct object pronoun los is used to replace it in the second sentence.