Plural of compound nouns
 

I assume the plural of "hijo de puta" is "hijos de puta" but is the plural of "hijo de su madre" then "hijos de sus madres" or just "hijos de su madre"?

I'm still not quite clear when you have to put all words in plural and when NOT. For example, it's "coches policía" but "montañas rusas", no?

Is there a specific rule here?

Hi, Manuel,

In the noun phrase "hijo de su madre", the noun is "hijo" and "de su madre" is a noun complement that specifies the "hijo" we're talking about. Though "de su madre" does function as an adjective, as a noun complement it doesn't undergo inflection like a regular adjective.

In the case of "hijo de su madre", we're talking about an "hijo" that belongs to a specific person, "su madre", and the head noun "hijo" is the only word which carries the number of the noun phrase. Consider if we said "hijo de Rosa". If Rosa has more than one hijo and we wanted to specify them, we'd say "hijos de Rosa", not "hijos de Rosas", as we are still talking about children of one person, Rosa, not multiple people named Rosa. In the same way, the plural of "hijo de su madre" is "hijos de su madre".

As for "coches policía", it looks like a simplification adopted over time of "coches de policía", another noun phrase which follows the same behavior I outlined above. As such, in the original phrase "coches de policía", the word "policía" would stay singular and the abbreviated form "coches policía" simply inherits this behavior. On the other hand, "ruso" is a regular adjective and so it must agree in number and gender with the noun it modifies.

Thanks for the explanation cloudgazer! it was very clear.