"I would like to buy fruit at the supermarket."
I've seen several instances when Duolingo used both characters in one sentence. I had the impression that, when combined together, they would mean "would like", but I don't know if I made the right assumption. I'm still confused as to how you would show inflections of a verb in Mandarin Chinese, like how do we know that 想 means "would like" or when it just means "like"?
That is the real answer, though. The grammar in your sentence is wrong. 在超市 must go before the verb. Place prepositional phrases with 在... must go before the verb they modify in almost all cases. There are a few verbs that are exceptions to this. 住 and 坐 are the only exceptions that I know, but there are probably a few more.