A preposition is a word used to show a relationship between a noun or pronoun and the rest of the sentence. Prepositions may indicate relationships such as time, movement, or location. A preposition usually begins a prepositional phrase which may function grammatically as an adjective or an adverb.

Jack is the man with the dark hair.

The preposition with starts the prepositional phrase with the dark hair. The noun hair is the object of the preposition. The entire prepositional phrase is functioning as an adjective describing the noun man.

Sarah and Becky walked to class this morning.

The preposition to starts the prepositional phrase to class. This phrase functions as an adverb modifying the verb walked to tell where Sarah and Becky walked.

Since there are so many prepositions that can do so many different things, the best way to recognize them is to simply be familiar with a good list of common prepositions.

aboard	as	by	in	onto	save	unlike
about	at	concerning	inside	opposite	since	until
above	before	considering	into	out	than	unto
across	behind	down	like	outside	through	up
after	below	despite	minus	over	throughout	upon
against	beneath	during	near	past	till	versus
along	beside	except	next	per	to	via
amid	between	for	of	plus	toward	with
among	beyond	following	off	regarding	under	within
around	but	from	on	round	underneath	without