shortening of the adjectives

[Randall]

can anybody help me out with the rules for shortening the adjectives? Like when you have ninguno and then exactly when you shorten it to ningun? Thanks in advance.

[gramatica]

Hola:

You shorten the adjective before a masculine singular noun.

For example,

You say "Algún día"=some day and you don't say "alguno dia"

The rule:

This usually happens with "bueno, alguno, ninguno, primero."

Examples:

El primer día de clases
¡Buen ejemplo!
Ningún lapiz

Espero que te sirva
Saludos