Preposition

A preposition is a word which typically goes before a NOUN PHRASE or PRONOUN. Let's see some examples below:
(1) of the world
(2) with my best friend
(3) by us
(4) at a hotel

1. Common prepositions

Here is a list of common prepositions. The words in bold are the most common.

about	around	beyond	into	over	toward(s)
above	as	by	like	past	under
across	at	down	near	round	until
after	before	during	of	since	up
against	behind	for	off	than	with
along	below	from	on	through	within
among(st)	between	in	onto	to	without

Many less common prepositions are written as two or three words. We call these complex prepositions. Let's see some examples below:
(1) because of
(2) by means of
(3) in addition to
(4) instead of
(5) on top of
(6) other than
(7) out of
(8) such as
(9) up to

2. Position

As its name tells us, a preposition is normally "placed before" a noun phrase or some other elements. The preposition + noun phrase together form a PREPOSITIONAL PHRASE.

2.1. In speech and informal writing

Sometimes the preposition goes at the end of a clause or sentence.

Social usage: the preposition at the end is common in speech and informal writing. But some people regard it as "more correct" to put the preposition at the front of the clause. This is possible for (I) and (II) in 2.2. below.
(I) For whom is she working? [formal]
(2) Town in which he was born. [rather formal]

But the preposition at the front is common only in formal writing. In general, do not be afraid to put the preposition at the end.

2.2. In different kinds of sentence

In (I)-(VIII) below, the first example in each section shows the preposition at the end of the sentence, and the second example shows the preposition in its usual position, at the front of its noun phrase.
(I) Question: Who is she working for? | She's working for a friend.
(II) Relative clause: the town (that) he was born in. | He was born in the town of Texas.
(III) Indirect question: I wonder which team he plays for. | He plays for the home team.
(IV) Exclamation: What a terrible situation she's in! | She's in a terrible situation.
(V) Passive: He's being well looked after. | They're looking after him well.
(VI) Comparative: He's been to more countries than I've been to. I've been to fewer countries.
(VII) Infinitive: This pen is difficult to write with. | It's difficult to write with this pen.
(VIII) Emphatic WORD ORDER: Some games I'm quite good at ... but I'm hopeless at golf.

I hope this article helps. If you have any question, please leave a comment below.