Prepositions are connecting words used to show the relationship of one word to another.

The word preposition means placed before. It is so named because it is usually placed before a noun or a pronoun or a noun phrase or a clause.

Prepositioner har som uppgift att ange en relation mellan ett substantiv eller pronomen och ett annat ord.

Prepositionen är vanligen placerad före substantivet.

Prepositioner anger ofta riktning eller befintlighet/läge för ett substantiv.

Prepositionsexempel - Sample Prepositions:

  av, för, åt, till, från, mot, längs, ovanför, nedanför, bakom, på,  i, under, över/ of, along, above, below, from, toward/-s, behind, for, to, on, in, under, during, over etc.

OBS! prepositioner används mycket olika i olika språk. Även om ibland "på" kan motsvara "on" så kan man icke räkna med att det skall vara "on" i engelskan där man i motsvarande position i svenskan använder "på"! 

-Denna omständighet kan aldrig tillräckligt betonas!

Exempelvis - For example: 
interested in= intresserad av
be on time= vara i tid
come on Friday, at noon= kom på fredag, på midda'n
a head-on collision= en frontalkrock

IMPORTANT
Det finns enkla och sammansatta prepositioner. Ibland hänvisas i grammatisk facklitteratur till 'Single' or 'Simple Prepositions' & 'Compound' or 'Complex Prepositions'. Har du för avsikt att ägna särskild tid och kraft åt studier av prepositioner så bör du vara bevandrad i terminologin alternativt du kommer inte att få ett tillräckligt underlag för en fördjupning i ämnet. Svensk litteratur i ämnet engelska med inriktning på prepositioner är inte alltid särskilt uttömmande. 

There are single (simple) and compound (complex) prepositions. Make sure you are aware of the somewhat confusing terminology. See below! Complex (compound) prepositions consist of two- or three-word combinations acting as a single unit.
Look at these 2-word combination examples: according to, along with, apart from, because of, contrary to, due to, except for, instead of, prior to, regardless of.
3-word combinations  might look like this: Single (Simple) Preposition + Noun + Single (Simple) Preposition.
in aid of, on behalf of, in front of, in accordance with, in line with, in relation to, with reference to, with respect to, by means of. These combinations also come before a noun.   

In some grammars they like to describe the types as being of basically three (3) kinds: simple, compound and phrasal.

Simple: at, in, of, on
Compund: alongside, throughout, underneath
Phrasal: across from, in place of, near to  
 
It is important also to grasp the fact that a prepositional phrase (incl prep, the object of the prep, and the modifiers of the obj)  may function as an adverb or an adjective. 

The main thing here is that you really understand the role and the function of the preposition by and large.

Then there is of course the category mostly referred to as Marginal Prepositions. They have affinities with other word classes. Some are in verbal form: 
Following her resignation she moved to the country.
The whole team was there, including Debbie and Karen.  

Other marginal prepositions are: concerning, considering, excluding, given, granted & pending.

Non-verbal marginal prepositions include worth (it's worth three dollars) and minus (twenty-three minus twenty  is three)

in the morning= på morgonen

in the evening= på kvällen

MEN  at night= på natten

at noon= på midda'n (kl 12:00 am)

vänta på= wait for

wait on= servera/betjäna bord, matgäster etc.

-Jämför - Compare "Jump in the water" med - with "Jump into the water" and then translate into Swedish!

Finally, there is one more thing having to do with prepositions and terminology:  Stranded Preposition, meaning it is placed somewhat off from it's more regular place ahead of and before the noun: "That is the girl Vince talked about."