A preposition is word used to describe the relationship between other words in a sentence.
Prepositions are almost always combined with other words. In grammars, these structures are called prepositional phrases.
A prepositional phrase consists of a preposition followed by an article or another determiner and an adjective or two, followed by a pronoun or noun. Here the adjective modifies the noun or pronoun which acts as the object of the preposition.
A prepositional phrase acts as an adjective or an adverb.
Ending a sentence with a preposition
You may have heard that ending a sentence with a preposition is a serious breach of grammatical rules. Well, not really. Of course, you are not supposed to end every one of your sentences with a preposition, but sometimes ending a sentence with a preposition is better than moving it to another location in the sentence.
Consider the two examples given below.
1) The professor asked the students to indicate the reference book they are quoting from. (This sentence ends in a preposition.)
2) The professor asked the students to indicate from which reference book they are quoting.
As you can see, sentence 2 isn’t much better than sentence 1 although it doesn’t end in a preposition.
The prepositions in, at and on can all indicate position. Here is a list of common word combinations with these prepositions.
In the bed / in the bedroom / in the car / in the class / in the library / in school
At class / at home / at the library / at the office / at school / work
On the bed / on the ceiling / on the floor / on the horse/ on the plane / on the train
As you can see, with some of these locations, different prepositions are possible.
No prepositions are used with the following expressions: downtown, downstairs, outside, inside, upstairs, uptown etc.