An ELT Notebook : Prepositions / Prepositional Phrases / Prepositional complement / Prepositional Object

1. A preposition is a word class which, as its name suggests,  cannot stand alone. It occurs in "pre-position"  to some other item which acts as prepositional complement (also called prepositional object) and completes the prepositional phrase.

Prepositional complements may be any of the following (preposition in bold, complement underlined):

a) A Noun Phrase

We're going on Tuesday, probably in the afternoon.
Stop looking at me like that!

b) A V-ing clause

I found out by googling it

c) A wh- clause 

We were astonished by what we saw

d) Adverbs

We watched from above.
I can't do it by then.

In some constructions the prepositional complement may occur earlier in the sentence, but can always be retrieved. Eg :

That's the book I was looking for    =    I was looking for the book.

2.  Prepositions may be simple - ie composed of one word (at, by, on, over, with, despite etc)  or complex - ie composed of more than one word (in spite of, away from, because of). As these three examples show, complex prepositions generally fall into one of three categories :

a) Preposition + noun phrase + preposition : as a result of, in view of, in line with etc

b) Adverb + preposition : apart from, out of, as for etc

c) Verb or adjective or conjunction + preposition : regardless of,  owing to,  because of   etc

3. Prepositions may also be correlative - ie composed of two parts. For example from/to, between/and

Further reading:

Quirk et al, (1985) A Grammar of Contemporary English. Longman  Chapter 6

and from An ELT Notebook...

An ELT Glossary : Multiword Verbs
Prepositional Verbs and Verbs with Prepositional Phrases. What's the Difference?