ajguin or algo?

Hola Sandra

When we say you can't use algo with a noun, we are referring to a noun right after, i.e with algo acting as an adjective. At the end of the lesson we have this example to illustrate how algo can't be used with a noun: 

Prefiero algo libro de aventuras. (incorrect)

Prefiero algún libro de aventuras. (correct) 

I prefer some adventure book.

You can say for example: 

Quiero algo para comer.

I want something to eat.

Quiero algún plato rico para comer.

I want some tasty dish to eat.

But you can't say:

Quiero algo plato rico para comer.

I hope these examples clarify the difference.

Saludos

Inma