Learn German: Ich vertraue ihnen nicht.

The verb vertrauen (to trust) is a special verb in German that always requires its object to be in the dative case. Instead of the regular accusative word sie (them), we have to use the dative pronoun ihnen.

Could ihnen also mean "you" in this sentence?

It sounds exactly the same when spoken, but in writing you can tell it means "them" because it starts with a lowercase i. If it meant the formal "you", it would be capitalized as Ihnen.

Why does nicht go at the very end of the sentence?

When you want to negate the entire action of the sentence, which in this case is the act of trusting, nicht usually goes at the end. It naturally follows the dative object ihnen.

How would I change this to say "I don't trust him"?

Since vertrauen needs a dative pronoun, you just swap ihnen for the dative word for "him", which is ihm. The sentence becomes: Ich vertraue ihm nicht.