The word pairs because of and due to are not interchangeable. The reason they are not is that they grew up differently in the language.
Because of grew up as an adverb; due to grew up as an adjective. Remember that adjectives modify only nouns or pronouns, whereas adverbs usually modify verbs. (The fact that adverbs occasionally modify other adverbs or even adjectives and entire phrases is not relevant to this particular discussion.)
To be more precise, with their attendant words, due to and because of operate as adjectival and adverbial prepositional phrases. To understand how the functions of due to and because of vary, look at these sentences.
1. His defeat was due to the lottery issue.
2. He was defeated because of the lottery issue.
In sentence #1, his is a possessive pronoun that modifies the noun defeat. The verb was is a linking verb. So, to create a sentence, we need a subject complement after the verb was. The adjectival prepositional phrase due to the lottery issue is that complement, linked to the subject by was. Thus, it modifies the noun defeat.
But in sentence #2, the pronoun "he" has become the sentence's subject. The verb is now was defeated. As reconstructed, He was defeated could in fact be a complete sentence. And due to has nothing to modify. It's an adjective, remember? It can't very well modify the pronoun he, can it?
Neither can it refer to was defeated because adjectives don't modify verbs. Sentence 2, therefore, should read: He was defeated because of the lottery issue. Now the why of the verb was defeated is explained, properly, by an adverbial prepositional phrase, because of.
In informal speech, we probably can get by with such improper usage as His defeat was because of the lottery issue, and He was defeated due to the lottery issue. But we shouldn't accept that kind of sloppiness in writing. We don't want to look stupid among those in the audience who know better. If we show them we don't care about the language, how can we expect them to believe us when we tell them that we care about the facts?
OK, how well do you know it? A little practice makes perfect. Click here, if you're game.
Updated Aug. 12, 2009