Diacritics are usually forgotten; however, they are extremely important in order to learn Arabic pronunciation. Diacritics (تشكيل), pronounced as Tashkil, literally means Forming, which is its main function, since Arabic letters on their own, without Tashkil, do not show proper guidance as to the proper Arabic pronunciation. Therefore, learning diacritics goes hand in hand with learning Arabic.
If your interested to learn more check this article: Learn Arabic Diacritics: Tashkil
Hamza does not originally belong to the original 28 Arabic letters. It is, however, derived from the Arabic letter Aein (ع). It is most often seen atop or below the letter.
But it is still considered one of the main vowels in the Arabic language.
If you want to know more about the Hamza and its forms, try to check on this article Learn Arabic Letters and Diacritics: Al-Hamza