"I am going to the shop"
anytime the suffix "-in" is added to a noun it confers the meaning of a preposition. so "dukani" would be "to the shop" whereas "duka" is "shop". The exception to this is proper nouns, i.e., you would not say "tanzaniani" (adding the ni), you just would leave it Tanzania. Maduka is plural of duka.
Why wasn't this taught in the lessons proper? I do the lessons, read the notes and then cannot do the reviews. I'm about to give up on learning this language here.