Demonstrative pronouns are used to demonstrate (i.e., make explicit) who or what is being referred to in an utterance. Similar to personal pronouns, they can replace a previously mentioned noun (or noun phrase). Unlike personal pronouns, however, demonstrative pronouns place additional emphasis on the person or thing being referred to.
Wer hat in meinem Bettchen geschlafen?
Who slept in my bed?
Wahrscheinlich dieselbe Person, die in meinem Bettchen geschlafen hat!
Probably the same person who slept in my bed!
Aber wo steckt diese Person jetzt?
But where is this person now?
Die ist in meinem Bettchen!
She's in my bed!
The demonstrative pronouns include the definite articles der, die and das, as well as:
this (the former)
that (the latter)
Demonstrative pronouns may refer to a person or thing, idea or place that has already been discussed, as in the following example:
Kennst du das Mädchen?
Do you know the girl?
Nein, das habe ich noch nie gesehen!
No, I've never seen her! *
* Nota bene: remember that das Mädchen in German - as unfair Snow White finds this - is still grammatically neuter!
Der, die and das
In spoken German, the definite articles der, die and das are often used as demonstrative pronouns because they lend more emphasis to the referent than the personal pronouns er, sie and es.
These demonstrative pronouns agree in terms of gender, number and case.
Selber and selbst are invariable, meaning they are not declined.
Darf ich das Bett machen? Hatschi!
Could I make the bed? Achoo!
Überhaupt nicht - du niest zu viel. Ich mache das Bett selber.
Absolutely not - you sneeze too much. I'll make the bed myself.
Derjenige and derselbe are declined in a two-part fashion. The first part (i.e., der) is declined like the definite articles above, while the second part (i.e., jenige) is declined like the weak adjective endings.
Gern gäbe ich demjenigen, der das Mädchen verlassen hat, eine zünftige Tracht Prügel!
I would gladly give the ones who left the girl a beating!