Here is a simple sentence with a deceptively simple mistake: I spoke to her yesterday for awhile. The problem is common enough that it often passes unnoticed, but something is stirring in the concluding prepositional phrase. There is the old adage that writing is rewriting. Let’s apply the adage.
A phrase, we remember, is a group of words without a subject and verb, and we generally name a phrase by the kind of word with which it begins: a prepositional phrase, then, is a group of words without a subject and verb that begins with a preposition—just what we have in the phrase for awhile. The preposition, though, brings its own demands into its construction, chiefly that it must have an object of some sort, generally a noun or pronoun, to which it points and which brings the phrase to a close. Earlier in the sentence we find the prepositional phrase to her, to being the preposition and her, a pronoun, its object. Prepositions show the affiliation or connection between objects across a sentence, so here the preposition to intends to connect the word her with I, the subject of spoke.
But what is the object of the preposition for in the phrase for awhile? Or to ask the same question another way, what’s an awhile? The object of a preposition must be a substantive, something, whether concrete or abstract, that can be named. No such thing as an awhile exists, so herein lies the heart of the problem: a prepositional phrase that has no object. A while, however, is something that does exist. A while is a short period of time, and as a noun, it can stand as the object of a preposition. We have, then, the answer to our problem: the phrase for awhile should be corrected to for a while. The preposition for now has a proper object and all’s fine in the world.
The problem here is caused by the fact that the word awhile does indeed exist in English. Awhile is an adverb that means for a while, so when we write for awhile, we are overlapping the same meaning. We really have, then, two choices: either I spoke to her yesterday for a while or I spoke to her yesterday awhile. Grammatically, both are acceptable, but the first, I would judge, is the preferable for reasons of euphony: the five syllables of yesterday awhile seem to bump and jostle one another roughly; they’re a mouthful, and they can present themselves a little more elegantly with another syllable in the grouping: yesterday for a while.
The point to make, then, is that we are served well when we look at our revision work in two parts: form and syntax. Form means identifying what we are analyzing as a word, phrase, or clause; syntax means finding the part of speech that form is acting as. The prepositional phrase for a while is as a phrase an adverb, so that’s one choice. But awhile is as a word an adverb meaning the same thing. That’s another choice. And as my grandmother with her box of chocolates used to tell me when I was a boy: just one at a time.