Awamil Un Nahw descripción

Nahw: It is most often translated as ‘grammar’. Nahw is a study of the language and the various rules governing the words as they appear in a sentence. For example I will now mention to you three sentences and discuss the difference between them please pay close attention.
1. ‘la tashrubil-laban wa ta’kulu as-samak’
2. ‘la tashrubil-laban wa ta’kulis-samak’
3. ‘la tashrubil-laban wa ta’kula as-samak’
What is the difference between these three in meaning? The difference between them is in the ending of the verb ‘ta’kul’ which means to eat. In the first sentence ‘ta’kul’ ends with a dummah.