Learn Danish: Hun spørger om vi ikke forstår sandheden. - She asks whether we do not understand the truth.

This is the golden rule of Danish subordinate clauses! In a normal, standalone sentence, ikke comes after the verb (vi forstår ikke). But because om vi ikke forstår sandheden is a subordinate clause (a dependent sentence attached to the main one), the rule changes: adverbs like ikke must be placed before the verb.

Could I use hvis instead of om here?

No, because they mean different things. You use hvis when "if" sets up a condition (like "if it rains, I will stay home"). When "if" means "whether", as it does here after a question word, you must use om.

Does spørger om mean "asks about"?

It can, but not in this sentence. When followed by a noun, spørger om means "asks about" (e.g., hun spørger om hunden = she asks about the dog). But here, om acts as a linking word introducing a new clause, so it translates to "asks whether" or "asks if".

How do we get "the truth" from sandheden?

In Danish, the definite article ("the") is usually attached to the end of the noun. The base word for "truth" is sandhed, which is a common gender (en) word. To say "the truth", you just add -en to the end, creating sandheden.