Theorie:

Pronouns
Pronouns are words which replace full nouns. Concerning personal pronouns we differentiate between subject and object pronouns. The following two tables present an overview:
 
SUBJECT
Singular
Plural
1st person
I
we
2nd person
you
you
3rd person
he
she
it
they
 
OBJECT
Singular
Plural
1st person
me
us
2nd person
you
you
3rd person
him
her
it
them
 
Beispiel:
Cindy owns a red bike. It is her favourite possession.
 
Hey, Joe, Alison and I are going to the cinema. Would you like to join us?
 
The possessive pronoun does not only replace the noun, but also indicates to whom it belongs. The following table provides an overview:
 
POSS. PRONOUN
Singular
Plural
1st person
mine
ours
2nd person
yours
yours
3rd person
his
hers
its
theirs
 
In case you don't want to replace the noun but indicate its possession, you would use a possessive adjective. (see table)
 
POSS. ADJ.
Singular
Plural
1st person
my
our
2nd person
your
your
3rd person
his
her
its
their
 
Beispiel:
I finally saved enough money to buy this new collectible. I still cannot believe it is mine.
 
She likes her boyfriend's parents a lot. She would trade them for hers in a heartbeat.
 
We would love to come to your party, but we have already been invited to another party by our neighbors - we will probably attend theirs.
 
The reflexive pronoun is used to express that somebody is doing something on his/her own. The following table provides an overview:
 
REFLEXIVE P.
Singular
Plural
1st person
myself
ourselves
2nd person
yourself
yourselves
3rd person
himself
herself
itself
themselves
 
Pay attention that the ending -f turns into -ves in the plural forms!
 
Beispiel:
When I look at myself in the mirror, I see different people, depending on my mood. What do you see, when you look at yourself?
 
Brace yourselves - the earthquake is not over yet!
 
I do not understand how these people can live with themselves. If I was in their place, I would have a guilty conscience.
 
The relative pronoun is used to introduce a subordinate clause which refers to the previous noun phrase (or to the whole sentence). While who and whom are used for people, which is used for things or situations. The following table provides an overview:
 
Subject
Object
Possessive
who
who(m)
whose
which
which
whose
that
that
 
 
 
Relative pronouns are used after a noun phrase:
 
1) to clarify which person or thing is meant.
e.g. I know the girl who lives next door.
 
2) to give additional information. >> Attention! In this case you need to add a comma.
 e.g. I know Sarah, who lives next door.
 
Apart from that, relative pronouns can also be used at the end of a sentence and, consequently, refer to the whole sentence. (Don't forget the comma!)
e.g. I travelled to Italy last summer, which was a great experience.