Learn Portuguese (Portugal): Não o posso destrancar porque o cadeado está estragado. - I cannot unlock it because the padlock is broken.

In a verbal complex like posso destrancar (auxiliary verb + infinitive), a pronoun typically goes after one of the verbs. However, the negative word Não acts as a magnetic word. It pulls the pronoun to the very front, forcing it before the auxiliary verb posso.

If the sentence were positive, how would the placement of o change?

Without Não pulling it to the front, you would have two options. You could attach it to the auxiliary verb (Posso-o destrancar) or to the infinitive (Posso destrancá-lo). Notice as a quick refresher that attaching o to an infinitive drops the verb's final -r and changes the pronoun to -lo.

Are the two instances of o in this sentence doing the same job?

No, they are entirely different types of words. The first o in Não o posso is a direct object pronoun meaning "it," which takes the place of the padlock. The second o in o cadeado is simply the definite article meaning "the."

Why do we use está instead of é for a padlock that is broken?