Learn Dutch: Ik ben nooit kwaad op mijn broer. - I am never angry at my brother.

In Dutch, being angry at someone uses a fixed preposition: kwaad op. Just like in English you have to memorize that you are angry "at" or "with" someone, in Dutch you are always mad "on" someone. It is best to memorize the combination kwaad op as a single chunk.

I previously learned boos for angry. Could I use it in this sentence instead?

Yes, absolutely! Boos and kwaad both mean angry or mad, and they are mostly interchangeable. Conveniently, they also share the exact same fixed preposition, so you can perfectly say Ik ben nooit boos op mijn broer.

Why does nooit come immediately after the verb ben?

In a standard Dutch sentence, adverbs of time like nooit (never), altijd (always), or vaak (often) typically come right after the conjugated verb. This gives us the standard word order: Subject (Ik) + Verb (ben) + Time adverb (nooit).