"Nel" and "nello" BOTH mean "in the". The difference is the letter (or letter combo) the subsequent noun starts with. So if the noun starts with "s", "z", "st" or two or three other letters or letter combos, you have to use "nello", but if it starts with most other consonants, you use "nel". However, gender is also a factor, so if the noun were feminine, then you'd say "nella", regardless of the initial consonant.
There are very similar rules for the definite articles and other combinations of prepositions and definite articles. Have you read any of the Tips and Notes for this course? A lot of this stuff is explained in those. Even if you only do Duolingo on your phone, the Italian course should include Tips and Notes (a lot of languages on the mobile app still don't have Tips and Notes, but the really major ones do, and on the PC version of Duolingo, ALL languages have T & N). I highly recommend a good study of Tips and Notes (which are attached to most Skills). That should clear up a lot for you. But in the meantime, I hope this answer has helped a bit.
There actually is, but it takes some preparation and has nothing to do with Duolingo. Also this method assumes you use Windows. I don't know about other systems. So:
First, go to your Start menu and search "character map". Open the program, it's just a little box full of characters. Find the one you want and select it (you have to click it, then click select), then it will appear in the field at the bottom. From there you just copy it, and as long as you have it copied you can just hit Ctrl+V to whack it into the middle of your sentence :D
Of course, you can only do this for one character at a time, so I just use è as it's the most common. I hope this helps, it certainly speeds things up for me :)
Why can't the fly be ON the sugar? It makes just as much sense. What if the sugar was in a pile, and the fly landed on it?
That would be 'sullo'. A fly could've dug into the sugar and be in the jar, under several layers of sugar, therefore "nello" zucchero.
Many many years ago, when I was a kid, one could buy sugar cubes that contained a fake plastic fly (for entertainment reasons,I suppose). Hence la mosca era nello zucchero :-)