German Grammar Explained /
Indefinite pronouns as companions
Some indefinite pronouns can be used only to “stand in” for other words. They become the subject of the sentence they are in.
Hat jemand meine Brille gesehen? Has someone seen my glasses?
Nein, niemand hat deine Brille gesehen. No, no one has seen your glasses.
There are also some that can be used alongside a name, as an adjective. These are the known as accompanying or companion indefinite pronouns.
Hast du einen Stift für mich? Irgendein Student hat meinen genommen. Have you got a pen for me? Some student took mine.
Ich habe leider keinen Stift für dich. Unfortunately, I have no pen for you.
Another example:
Ich habe Hunger. Ich habe heute noch nichts Richtiges gegessen. I am hungry. I have not eaten any proper food today yet.
Gut, dann kochen wir uns etwas Leckeres. OK, let's cook something tasty.
These are very common in German, so you're going to encounter them often ;).