What is the difference between a preposition and a conjunction? The question can help us see that writing a good sentence—and understanding what we’ve written—involves not just finding the right words, but combining those words into meaningful groups. Being able to see word groups dissolves many a grammatical problem, and can even suggest new revisions to a sentence that may not be saying what we want to say in just the right way.
Both prepositions and conjunctions fall under a broad grammatical category called connectives, but each has a defined role to play in building out a sentence. Whether a word is a preposition or conjunction will often depend on the other words with which it is working. Grammar is contextual, which means the same word can function as a preposition in one sentence and as a conjunction in another. Prepositions show the relationship, or connection, between one thing and another, whether that relationship is literal (a pencil on the table) or figurative (a lecture on gardening). Conjunctions, like the word and, for example, also show a connection, but the relationship can’t be physical: to say pencil and paper is to associate two things, a pencil and some paper, in our mind, not in relation to something else in the physical or figuratively physical world.
We can see the difference between these two parts of speech more easily if we keep in mind the rules that oversee their composition. A preposition must have an object, most often a direct noun: in the group of words on the table, on is the preposition and table is its object, the two beginning and ending what is called a prepositional phrase. When grammar talks about an object, it means something other than something else. So in the prepositional phrase a pencil on the table, the preposition on intends to relate two isolated things in regard to their position to each other. But if I want to expand the idea and say that a pencil and paper were on the table, the conjunction and is indeed joining two things, pencil and paper, but it is joining them together as one conception: I am saying, in effect, “Think of these two things as one, and now relate that pair to something else in regard to position, namely, the table.” That, believe it or not, is all going on in our minds (thankfully unconsciously) in a sentence even as simple as pencil and paper are on the table.
But the plot thickens. As it happens, the same word can often function as either a preposition or a conjunction, given the other words it works with. Consider the word than, for example, in these two sentences: he has more than $10,000 in the bank and he has more in savings than I have. In the first sentence, than is a preposition, with $10,000 standing as its object (the word more is an adverb modifying the verb has), and it is connecting the money to the bank, two very different things. In the second sentence, though, the word than is a conjunction, which we can first see by the fact that it has no object: the words I have form a clause and represent an action, not a thing. And that is in keeping with what we have already observed about conjunctions: they connect ideas, not things. Here the conjunction than means to connect his savings and my savings, how much he has and how much I have. The sentence with the conjunction wants to talk about the idea of having savings, not the money itself, though that subtle difference will be dramatically realized were I to request a certain withdrawal.
The practical observation to make for revision, then, is that prepositions always have an object in the form of a noun. Preposition and object together form a prepositional phrase, and that phrase can often transform itself into a clause, helping us elaborate the idea for emphasis: he has more than $10,000 in the bank, and that’s a lot more than I have. It’s true that we generally try to reduce the number of words we’ve written revision to revision, but sometimes we need to extend an idea to carry its meaning through. Looking at the grammatical structure of what we’ve written can suggest possibilities that might otherwise go unnoticed, with the aim of making these changes more automatically (or unconsciously) after we’ve once become conscious of the structure of the language.