"Is Anybody Home?"
If you look up anyone or anybody in a bilingual dictionary, you're likely to find cualquiera and cualquier, as well as alguien and nadie. They are not interchangeable. Since most inexpensive bilingual dictionaries do not provide examples of usage, you could make a usage error. Let's tackle these with a few easy-to-remember examples and what they mean in English.
If you mean anyone at all, as in any ol' person, you need cualquiera, when used as a pronoun:
Cualquiera puede hacer esta tarea (Anyone/Anybody can do this task).
If used as an adjective, cualquiera becomes cualquier and goes before the noun it modifies, regardless of its gender:
Cualquier persona aquí te puede ayudar (Anybody here can help you).
The plural forms are cualesquier and cualesquiera:
Traiga cualesquier herramientas que tenga a mano (Bring any and all tools you have at hand).
Another solution, when the notion of anyone is applied with the sense of whosoever or whomsoever is quienquiera:
Quienquiera que oiga esto dirá que es una mentira (Anyone who hears this will say it's a lie).
Dígaselo a quienquiera, no me importa un bledo (Tell whomsoever you please, it doesn't matter a bit to me).
If you mean anyone in a question, as when seeking somebody, anybody, use alguien:
¿Hay alguien en casa? (Is there anybody home?)
However, as you probably know, there are negative forms for negative constructions. In such cases, anyone translates as nadie. Notice that when nadie is the antecedent of a adjective clause, the verb in that clause has to be in the subjunctive, because it modifies a non-existent antecedent (which is what nadie must mean, since it denotes no person):
No hay nadie en esta oficina que le pueda ayudar, lo siento (There is nobody in this office who can help you, I'm sorry).
Use extreme caution: If cualquiera follows a noun, it can be demeaning, particularly if used with reference to women:
Juana es una mujer cualquiera (Jane is anybody's/everybody's girl).