an adverb modifies verbs, adjectives, and other adverbs, often by telling how much or how often. In the case of 'truly', it is used to emphasize the veracity of something, as in 'It is truly remarkable.'
rules and use cases associated with truly in this capacity include adding emphasis to a sentiment (It was truly amazing), affirming a statement or opinion (She's truly my best friend), conveying intensity or seriousness (She was truly disappointed) and describing a situation objectively (It was a truly unique experience).
1. The chef truly outdid himself with this dish.
2. They were truly devoted to each other.
3. His techniques are truly revolutionary.
in speech and informal writing, the adverb form of 'truly' is sometimes used as an intensifier, as in 'It was truly, truly amazing'. However, this construction is usually considered incorrect in formal writing.