"The professor's pen is brown."
Translation:قَلَم اَلْأُسْتاذة بُنِّيّ.
Diacritics don't change the way words are written and the -i- here is just a Genitive mark. The only exception is adding an alif to indefinite singular nouns, adjectives and broken plurals in Accusative (unless they are diptotes), but then again it doesn't apply to words ending in a ة .
Thanks, PueoPulu. But what about when you want to add "your" (ik/ak) to a word. Isn't that adding a diacritic and a ك? So that if the noun ends in a consonant, you just add ik/ak (بَيْت becomes eg بَيتِك) But if it ends in a taa marbuta, the latter unravels so that eg مَدينة becomes مَدينَتِك, so why doesn't the same thing happen for genitive kasra?
It's not like you've written above that if we add anything to a taa marbuta it has to unravel - and certainly it won't be a vowel. It takes something bigger and permanent to make it untie and diacritics which come and go are simply no match for real additions influencing the shape of taa marbuta. The poor little kasra is just too insignificant on its own to make any difference in writing! (Fortunately, it's in good company of other diacritics so we don't need to feel sorry for it).
As you said, when we add "your", we add a diacritic (variable) AND a ك, and it is the ك (or any other possessive pronoun or suffix for that matter) that is a gamechanger for our spelling and unravels taa marbuta (NOT a diacritic preceding it). The ك et consortes may sound like innocent one-syllable appendages to words, just like diacritics. But we shouldn't be misled by their innocent looks! We should perceive them as fully-fledged words, only written slightly differently, attached to something, but unlike diacritics ALWAYS written. In fact, they are so important that they can have their own diacritics (the same that we can skip anyway)!
So, when مدينة becomes مدينتك it becomes a completely new word and ك must be obligatory rendered in speech, while vowels around it will vary or even be absent: hence it would be impossible to change shape of the letter because of a vowel that may or may not be used. Actually, it would undermine the whole concept of abjad, I think. الأُستاذةِ from the example has kasra marked only for our convenience, we don't really add anything. We can read it aloud or skip it, but the word remains the same الأستذة . Again, if we change it to أستذتك it won't matter whether we vowel it أستذتُكَ, أستذتِكَ, أستذتُكِ or anything else. The unraveld taa is there to stay.
I hope I managed to get my point across :)