Why do some adjectives use "de" and some do not?
I understand that the rule says that an adjective comes after the noun in Spanish (well, usually anyway). For example, "The red house" becomes "La casa roja". What I do not understand is the use of "de" sometimes being used and sometimes not. Why isn't "water glass" written as "vaso agua", rather than "vaso de agua"? I can follow the usage when "John's house" is written as "Casa de John" because of personal pronouns not being used the same way in Spanish, but I do not understand the use for something like water glass which doesn't seem to imply ownership. I'm certain that there is an easy explanation from someone though. Thanks in advance.
Water is not an adjective. John is not an adjective, nor a personal pronoun.
In English we often use a noun as an adjective- in Spanish when you want to modify something using a noun, you must use a connector (preposition) most commonly de
When something is truly an adjective you do not.
Watery is an adjective but water is not. Agua is a noun, not an adjective.
One could argue that vaso de agua actually could imply that the glass is made of water, and that to match water glass you should say vaso para agua and that for glass of water maybe vaso que contiene agua or vaso con agua adentro or vaso lleno de agua but no one does that, and the meaning is usually obvious from context.
There are occasional exceptions where two nouns are next to each other without such a connector- for instance rush hour- hora pico or hora punta.
In this case hora, pico, punta are all nouns.
When it is in plural it becomes horas pico or horas punta the second part is not pluralized the way it would be if it were an adjective.
These are not the norm however, and generally must be learned as a set phrase.