One Farsi word for garden is بوستان /bustän/ , which is, according to Moin dictionary, of Pahlavi origin. It was never adopted into Arabic.
That's cool. I never knew about that. I knew of Tajiki and Dari but never Pahlavi.
Some people have very good memories. I don't. In order to remember the prepositions you mention, I require the grammatical tables alongside exercises like we get here in DL. Then its a matter of practice, practice and more practice with repeated reviews of the grammatical rules. As soon as possible, start reading Swahili texts. I think the best answer is to combine all learning tools, both of a practical and theoretical nature and burn with zeal and love for learning!