Translation:Do you like to swim?
My Tanzanian Swahili teacher told me I was overusing "Je," in questions, which surprised me too, as other textbooks and online courses encourage it, e.g. https://issuu.com/igihe5/docs/swahili_grammar_for_introductory_an/170
That textbook suggests you use "Je," to alert the listener that you are about to ask a question. (Note: not only yes/no questions.) An equivalent that occurs to me in English is, "Tell me, what do you think of this course?"
In written text, the question mark at the end makes it obvious it is a question, so I can see why Duolingo - and my teacher - leave out "Je, ..." at the beginning in written exercises.
He also said it wasn't wrong in spoken Swahili, just unnecessary, as it would be obvious from the intonation. I found his question intonation hard to mimic though, and the other option he gave was to start with "Je, ..." and then speak with normal intonation.
Can someone out there give us the benefit of their experience about when they use "Je" before a question?
That would be a statement, not a question. You might be using question intonation, but it doesn't show when written - you would have to add a question mark. (Duolingo is very lax about question marks, but I would say that a question word is then essential to show that you have understood.)