What is the reason for the difference in the vocalization of the last letter of the word "الأرض" (the Earth) between the accusative case (fatḥah) in His, the Almighty’s, saying, "خلق السموات والأرض بالحق" (He created the heavens and the Earth in truth), and the genitive case (kasrah) in His, the Almighty’s, saying, "يعلم ما في السموات والأرض" (He knows what is in the heavens and the Earth)?

In the first verse, the word "الأرض" (earth) is in the accusative case (mansoob) with a fatḥah because it is conjoined with "السموات" (heavens), which is also in the accusative case, indicated by a kasrah in lieu of a fatḥah. Both are the object (mafʿūl bihi) of the verb "خلق" (He created). As for the second verse, the word "الأرض" is in the genitive case (majrūr) because it is conjoined with "السموات," which is in the genitive case due to the preposition "في" (in). The conjoined word (maʿṭūf) follows the grammatical case of the word it is conjoined with (maʿṭūf ʿalayhi) (whether nominative, accusative, or genitive). The word "السموات" remains in a single form because it is a sound feminine plural, which takes a kasrah for both the accusative and genitive cases.