This post is for those who love the Arabic Language.
In the beginning of Surah Al-Fatihah, after the Basmalah, we recite – Alhamdulillahi Rabbil Alamin, الْحَمْدُ للَّهِ رَبِّ الْعَالَمِينَ .
Sometimes people will ask, “Why does the word Lord [Rabb] end in a Kasra causing it to be pronounced Rabbi ربِّ ?”
Let’s look at the first part of the Verse.
1) Al-Hamdu is the subject [Mubtada] of the nominal sentence and so the word Al-Hamd ends in a Damma making it Al-Hamdu الْحَمْدُ
2) The Li in Lillahi للَّهِ is a preposition showing possession and it shows that the praise belongs to Allah, meaning He is the Owner of every Praise.
3) Now, here comes the word Rabbi [Lord] before the word Al-Alamin, and it, meaning Rabbi has a Kasra on the last letter because it is the Badal – Replacement noun for the word before it in this Verse, thus it would follow suit and take on the same marking-Haraka, in this case being a Kasra.
The reason why Lord [Rabb] does not have an Alif Lam in front of it is because of it being in the Idafa form and Allah always knows best.
I hope this helps, In Sha Allah.