I heard many people using the word Rabbul-Alameen. This article tells us to Reflect upon His saying, '(Rabbul-Alameen) the Lord of the Alameen (mankind, jinn and all that exists).' [Soorah al-Fatiha (1): 2] whereas v 1:2 is Alhamdu lillahi Rabbil Alamin.
So what is the difference between these two words? Should I address Allah as Rabbul-Alameen or Rabbil-Alamin in dua? Or are they synonyms of each other?
I can sense that it is more of a question on Arabic language itself than on Islam. But ArabicSE has been abandoned, and this particular question is not suitable for LinguiticsSE. Hence, I asked it here seeking suggestion.