Writers sometimes wrongly use a preposition to join the nouns in a compound subject, as in this example:
The bride as well as the groom drive off for their honeymoon.
The grammatical bond between “bride” and “groom” isn’t right here, and the culprit is the preposition “as well as.” Only the conjunction and can form a compound subject. To fix the sentence, replace “as well as” with “and”:
The bride and groom drive off for their honeymoon.
In the above example, the pronoun “their” signals that the writer intends to treat “bride” and “groom” as a compound subject. But in some cases the writer might not want to have a compound subject:
The mayor together with her advisers is drafting a new proposal for the town council.
The singular verb “is” indicates that only the noun “mayor” is the subject. In this case, a preposition is appropriate, but the prepositional phrase should be isolated by commas because it is not essential to the meaning of the sentence:
The mayor, together with her advisers, is drafting a new proposal for the town council.
Conjunctions and Prepositions Serve Different Functions
Conjunctions and prepositions serve different grammatical functions. A coordinating conjunction like and joins elements that are of equal weight in a sentence, such as two nouns or two independent clauses. A preposition, on the other hand, links sentence elements that are of unequal weight, and in doing so it shows the subordinate relation between the two elements (Cook 78).
When two nouns are joined by a preposition, the second noun has less weight than the first, and the prepositional phrase may be treated as an aside:
Page turning, along with dry skin, is responsible for my many paper cuts.
Although two nouns have contributed to this writer’s wounds, only one, “page turning,” is the subject of the sentence.
A Preposition Should Not Link Items in a Series
Items in a series are equally important, so a preposition should not link them:
At the intersection I was allowed to turn left, turn right, as well as go straight.
To fix this sentence, replace “as well as” with the conjunction “or”:
At the intersection I was allowed to turn left, turn right, or go straight.
Prepositions and conjunctions provide information about the relative importance of elements in a sentence, and misusing them can thoroughly confuse a reader. So check your own writing for inappropriate prepositions, and make sure that you’ve carefully weighed your parts of speech.
Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Houghton Mifflin Harcourt, 1985.