"Omar is Syrian."
Why in fully vowelled text is there a "kasra" below the "raa"? Since the "yaa" makes the "ii" sound, wouldn't the "yaa" be enough even in the fully vowelled texts? And, the "shadda", it doubles the "yaa", right? So, why is it there when the sound pronounced is "suuri"? From what we've been taught so far, it looks like the letter "yaa" and the "shadda" and "kasra" would spell suuri-ii-ii. ; )
In dialects, such ending of ـي (which is called the Relation Ya, and it is added to make adjectives out of nouns) usually goes smooth with the consonant preceding it; As in the example above, this would be: Sú-rí. However, in standard Arabic this is not the exact way to say it.
Such adjectives (and many others ending with ـي) are actually broken down with a silent or stable Y-sound, meaning Ya at the end has Sukún on it, so the word above for example should be: Sú-riy سورِيْ - this is the proper way to say it in proper Arabic.
However, this Sukún on the last Yá is not permanent because soon you will have to replace it with Shadda if you need to, for grammatical purposes, change the last vowel (e.g. add Tanwin like -un). You will have to double the Y sound and the word becomes: Sú-riyyun (a Syrian), or even if this has to change to plural: Súriyyún (Syrians). For this reason, for general purpose Shadda is added even though it should be Sukún there, but Sukún would be changed anyhow to something else once grammar plays with the word and doubling the Y-ending is a must (this is how the speech goes I'd say). All in all, this is in proper Arabic, and in dialects, we still would say -rí at the end here (and when in plural, the Y-sound is double anyway).