I have a little but not much knowledge of Arabic.
Although God is called
Al-Karim, I hear the invocation
Ya Karim, but I never heard the invocation
Ya Al-Karim or
Ya-l-Karim. Same for other names of God: the definite article disappears from the vocative. Just as it would in English:
May the Lord have mercy on us vs.
O Lord, have mercy on us
One big exception:
Ya Allah, which I've heard lots. (With definite separation between the end of
Ya and the start of
Allah, there are three different "a" sounds. I'm not talking about
yalla as in
Now I see that when my English translation of the Quran has
O God, the original is
Allahuma. I don't know anything like this construction, where someone is addressed by tacking "-uma" at the end or something similar.
Though as I say, I'm not at all knowledgeable in Arabic and might easily have missed something. So is there a difference? Is
Ya Allah in the Quran and I missed it? Is
Allahuma preferred? Is
Allahuma a unique construction?