Learn Dutch: De gids vertelt ons niet of het kasteel duur is. - The guide does not tell us if the castle is expensive.

This sentence contains an indirect question starting with of. In Dutch, indirect questions act like subclauses. This means the verb for that part of the sentence (is) must be moved to the very end of the clause.

Doesn't of usually mean "or"? Why does it translate to "if" here?

You are right that of often means "or". However, when you are reporting a yes/no question (like "He doesn't tell us if..."), of translates to "if" or "whether".

Could I use als instead of of to mean "if" in this sentence?

No, that is a common mistake for English speakers! Use als for conditions (like "If it rains, I will stay home"). You must use of for indirect questions where the original question was a yes or no question.

Why does niet come after ons and not before it?

In Dutch, the negative word niet generally comes after specific objects, including pronouns like ons (us). Therefore, we say vertelt ons niet rather than putting the negation before the pronoun.