It is my understanding that the following is correct:
Cette pomme est très rouge.
And this phrase can only be written with agreement between cette and pomme.
But there is also:
Ce sont nos fils, Pierre et Paul.
And now agreement is not allowed.
It is my understanding that the first phrase is using 'cette' as a definite article, while the second is using 'ce' as an indefinite demonstrative pronoun, and that this is the reason for the discrepancy (definite articles agree with the word with which they stand, indefinite demonstrative pronouns do not).
However, for me this does not make sense. In Portuguese, saying 'essa maça é ...', we'd classify 'essa' as a pronoun.
- Did I say anything false?
- what rule can I use that does not depend on morphological classification (or, equivalently, what rule can I use to create the morphological classification?)