Due was originally used only as an adjective, and therefore in formal English many writers use due to only to introduce an adjective phrase (one modifying a noun): Her success was due to hard work (modifies success). In informal English due to is often used [as a preposition] to introduce an adverb phrase: Due to her hard work, she succeeded. However, many people do not approve of this construction. To avoid it, use because of or on account of.