I know this is probably a stupid question, but in this sentence: “Quae pars orationis est ‘serenas’?” why is the word Quae instead of Quid? And I thought the signal “ae” was either first declension nominative plural or first declension genetive and dative singular.
Our text book (Artes Latinae) says the above sentence means, “What part of speach is the word serenas?”
Thanks!
The interrogative pronoun (quis, quid) is, when used adjectively, declined in the same fashion as relative pronouns. Therefore, Quis en Quid occur only substantively, undeclined.
Examples:
What man is wise? → Qui vir sapiens est?
What woman do you love? → Quam mulierem amas?
To whom should I give this? → Cui hoc dem?
With whom I should fight I don’t know → Quocum pugnare debeam nescio
“pars” is feminine. Therefore: quae
etc.
Clear?
To add to that, QUAE ends in AE because that is just its nominative form. While the form QUA does exist, it is not quite the same paradigm, so you just have to remember that QUAE is the interrogative feminine adjective (not pronoun) in the nominative.
Thank you so much! This really helps…
Why am I always using Qua as an int. fem. adj.? Why?
You shouldn’t infer, good sir. 
Yet, although I didn’t state so specifically, I would call pronouns, when used adjectively (hoc vir), adjective pronouns. Though in a sense incorrect indeed, as it, in that instance, stands not for a noun.
Although, perhaps I was inferring myself. 
As far as “qua” is concerned, it does ring a bell, in that, indeed, “qua” is not merely used as an ablative, but, if I recall correctly, sometimes also, as I believe benissimus mentioned shortly, as a nominative in an adjective sense. Often with “ali-” I believe. Surely, one could look it up in one of the grammars here. 