إِنَّ عَلَى الْمَكْتَبِ كِتَابًا
Translation: Indeed, there is a book on the desk.
«عَلَى»: preposition; «الْمَكْتَبِ»: genitive noun. The prepositional phrase is the fronted khabar (through an understood predicate); «كِتَابًا» is delayed accusative ism.
إِنَّ عِنْدَكَ قَلَمًا
Translation: Indeed, you have a pen.
«عِنْدَ»: accusative place adverb and muḍāf; «كَ»: genitive possessor; «قَلَمًا»: delayed accusative ism. The locative expression relates to the understood predicate.