In Noorani Qaida Page 8, we learn about double fathah, double kasrah, and double dammah that are known as Tanwen in Arablic. For instance, بً bann in this letter. The two fathah are above the letter Baa, then it is pronounced like “BANN”. When the Tanwen comes in Arabic letters  then its pronunciation would be changed. For example, as you can see in “كتابٌ” (Kitaabun), “كتابٍ” (Kitaabin), and “كتابًا” (Kitaaban), how the Tanween changes depending on the sentence. Moreover, another point is that there is also Ikhfa in this lesson. Ikhfa means when you pronounce Meem and Noon, the sound produced in the nose is known as Ikhfa. Nasal sounds are important for the learner.So, learn it perfectly.