Spanish language Q&A Forum

I'm not sure I understand why the usage is "mensajes clave" vs. "mensajes claves"

Is clave in this context an adjective?  If so, shouldn't it agree in number with the noun?
mensajes clave vs. mensajes claves

As an example, wouldn't  "important messages" be plural? e.g. "mensajes importantes"? 
Why does "clave" stay singular when "mensajes" is plural?

I must have missed this grammar lesson on adjectives like Clave :-)

My dictionary says fruit (the plant) is la fruta. Why is the sentence -Los frutos rojos  and not las frutas rojas ? 

 Al haberse portado tan mal, lo expulsaron del colegio.

the pronoun se came right after haber not after portado nor in front of haber.

This is different from any other rules I knew, like:

Me lo dijo. Me lo ha dicho. Dímelo ! Quiero dárselos.

So which rule applies to this ? 

In the quiz, I got the sentence

Antes de que tú digas nada, .... ( before you say anything )

Why is 'nada' here ? Can it be 'algo' ?

Another example from the other lesson is, though I don't remember the exact phase but it's like

No creo que hayan llegado todavía.

The original phase to be denied should be 'han llegado ya'. Again, why it changed to 'todavía' ?

I agree that whole expression has something negative, which hasn't happened yet. But I'm confused, because the phase in 'que' is totally affirmative.

So the expression in 'que' isn't independent from its use ? And how ?

I have a lot of problems with conjugation--past, past perfect, I am ok with the present tense.  Where can I learn more about how to conjugate verbs in all moods and tenses.