speech parts in Prepositional case are always preceded by preposition, which is what the case is named after

to think about is думать о/про so as a result you have думать о тебе where тебе is ты declined in the Prepositional case

насчёт in Hebrew would be 'be-noge'a le' or 'be-kesher el' so the difference is the same as between these prepositions and preposition 'al'