Why words in the Quran have different ending sounds?

Nouns are flexible meaning they have ending sounds and this ending sounds can change. Like Muslimun Musliman and Muslimin depending on the roles that they play. However, some nouns are more flexible than others. 

The discussions of flexibility only apply to singular nouns that are nouns that end with the sounds ‘un’ ‘an’ and ‘in’, If you hear aani or ayni or ooni or eena, you don’t have to worry about flexibility. 
Generally, flexibility can be divided into 3 categories first is fully flexible, secondly is partly flexible and thirdly non-flexible. 

Using our analogy of actors in a film or drama series, you can think of flexibility as an actor performing the lead role. Now they are not all treated equally even when they are all heroes in their own shows. The pay of newcomers to the industry is not the same as the pay for and established actor.

An award-winning actor has certain privileges that a rookie actor don’t enjoy. They might have their own cabin for example and most of the time, he can even change the script given to him. 

Fully flexible

Similarly, fully flexible words are like the award-winning actor they can change. This is their natural behaviour so there is nothing much to learn about fully flexible. Let’s take the word Muslim. If I want to make it rafa I say Muslimun, if I want to make nasb I say Musliman and if I want to make it jarr I say Muslimin. If want to make it light I would say Muslimu, Muslima, Muslimi.

Partly flexible

Then there are words that are only partly flexible, they do not enjoy the full flexibility. Let’s take the word Zainab. Under normal circumstances If I want to make it rafa I say Zainabun , if I want to make nasb I say Zainaban and if I want to make it jarr I say Zainabin.

But the word Zainab is partly flexible so the correct way of saying it is Zainabu, Zainaba and Zainaba. It cannot take the ‘n’ sound and ‘ee’ sound no matter whether it is light or heavy.  So the correct way to pronounce Zainab in the Quran it is either Zainabu for Rafa and Zainaba for both nasb and jarr. There is no such word as Zainabi. 

When is a word partly flexible?

How do you know when a word would be partly flexible? It is when they fall into two categories: Non-Arab names and places. Ironically in the Quran, there is only 3 prophets with Arabic names from a linguistic point of view and they are Muhammad, Salleh and  Shuib. 

The exception to partly flexible

There are two instances where a partly flexible word can become fully flexible. One is when there is Alif laam in front of it. Secondly when a non-Arab name or place is spelled with 3 letters.