Prepositions have the semantic feature that they allow us to locate nouns in time and space. In, on and towards all do this in the phrases in the cupboard, on the next day, towards the end.
Morphologically, they are generally short, often consisting of just two letters: in, to, of, at
Syntactically, they normally precede rather than follow nouns. I’m going to bed, and not *I’m going bed to. They can, however, occur at the end of a relative clause: The girl I gave my heart to.
Most linguists recognise complex prepositions, which consist of more than one word, such as because of, according to and as far as. However, as the ‘Longman Student Grammar of Spoken and Written English’ recognises
. . . there are borderline cases with complex prepositions. It is not always clear whether a multi-word combination is a complex preposition-that is, a fixed expression with a special meaning-or a free combination of preposition ( + article) + noun + preposition. At the expense of is an example of an in-between case.
I say ‘most linguists recognise complex prepositions’ because Rodney Huddleston and Geoffrey Pullum, the authors of ‘The Cambridge Grammar of the English Language’, don’t. They analyse by means of, for example, not as a complex preposition, but as a preposition, by, followed by a Noun Phrase, means of. They point out that ‘if by means of were really a single preposition, we wouldn’t expect to be able to insert similar after the first part of it and drop the last part to get by similar means.’
Huddleston and Pullum also ‘extend the membership of the preposition class beyond the words that traditional grammar calls prepositions’ because they ‘see no justification for restricting it to words that have Noun Phrase complements’. They argue that both before and know can have as their complements a Noun Phrase or a subordinate clause, or can have no complement at all, but where traditional grammar regards know as a verb in all three cases, it regards before as a preposition in the first, a subordinating conjunction in the second and as an adverb in the third. They ‘see this triple assignment as an unnecessary complication. It is much simpler to give before a uniform analysis, treating it as a preposition in all three, just as know is a verb in all three’.