A preposition links a noun or pronoun to the rest of the sentence, showing a relationship of place, time, or direction. In the examples below, each bolded preposition makes that connection clear.
She sat beside me. Here beside shows where she sat.
The meeting is on Monday. Here on shows when it happens.
Two quick scenes
The scenes below show how a preposition links a noun or pronoun to the rest of the sentence, expressing place, time, direction, or source. A quick source example first: She is from Kenya. Here from shows where she comes from.
She sat beside me. Here beside shows where she sat.
The meeting is on Monday. Here on shows when it happens.
Picture a delivery. She walked to the shop. The word to points at movement towards a destination, and the shop is the noun it connects her to. Towards works similarly, though it only stresses direction, so the shop may or may not be reached.
The café is beside the river. Here beside shows where the café sits, right next to the river rather than far from it.
We met after lunch. Here after shows when the meeting happened, in relation to lunch.
Picture a delivery. She walked to the shop. The word to points at movement towards a destination, and the shop is the noun it connects her to. Towards works similarly, though it only stresses direction, so the shop may or may not be reached.
Now a place and a time. The dog is under the chair. Here under links the dog to where it is; the chair, the noun right after the preposition, is its object. We met after lunch uses after to tie the meeting to a time.
Prepositions are easy to mix up with conjunctions, because some words, like before and after, can do either job. The next part shows how to tell them apart.
Not the same as a conjunction
One quick distinction worth keeping in mind: prepositions relate a noun or pronoun to the rest of the sentence, unlike conjunctions, which join whole clauses. Compare before dinner with before we ate. In the first, before is a preposition; in the second, it is a conjunction. The word after it tells you which job it is doing.