What is difference سومین or سوم in Persian

 There are a few options - answers the question:

1. سوم is a short form of the word سومین 2. سومین is similar to using article THE in English 3. سومین has the role of a noun

Comment:

both sets of examples are correct, the one with the stars * is used more than the other it that specific situation. او نفر سوم شد. او سومین نفر شد. روز سوم سال بود. سومین روز سال بود. *دخترم سالِ سوم دانشگاهه. دخترم سومین سال دانشگاهه. (if you want to use the second sentence its better to say دخترم سومین سال دانشگاهشه. ) تو نفر سومی هستی که این سوال را از من می پرسی. تو سومین نفری هستی که این سوال را از من میپرسی. (here pay attention to the "ی" added after the phrase نفر سوم and سومین نفر. it's as if we consider these 2 words as a whole, so we never add the ی in between.