Mass nouns (uncountable nouns) require a partitive article in French, and not in English, when the sentence is about an undefined quantity of that mass:
- je mange du sucre = I eat (some) sugar
If the mass noun is specified in the sentence, both French and English will use a definite article:
- the sugar that I am eating is white = le sucre que je mange est blanc
If the mass noun is inserted in a general truth, in French you will use a definite article and in English no article:
- sugar is naturally present in fruit = le sucre est naturellement présent dans les fruits
Now back to the sentence you are given here: "le sucre est dans le sac" is not a general truth (not all sugar in the world is in this bag), therefore this is a specific sugar: the sugar is in the bag.
If the French sentence were: "du sucre est dans le sac", the English translation would be "some sugar is in the bag", because "du" is partitive.
Literally, "au" (masculine singular) or "à la" (feminine singular) "aux" (plural) mean "at the" or "in the" or "to the".
You can use it for buildings, for instance "je suis [au/dans le] restaurant" (I am at/in the restaurant) or "je vais au restaurant" (I am going to the restaurant) or "j'entre dans le restaurant" (I am going in/I enter the restaurant).
"dans" means "in", and more specifically "inside" or "within".
The whole issue is that prepositions do not match exactly from one language to the other, so you have to learn constructions as they come.
The system compares your translation with what incubators have registered in the system.
In the system, we have registered such contractions as "I'm, you're, he's...". Not that they are required, since it is obviously proper English to write "I am, you are, he is..."; but they are very usual.
But we cannot register 's as correct to represent "is" after any noun used on Duolingo. The reason is simple: 's is also the mark of possession after a noun.
The same goes if you consider a form like "he has been here" that can contract to "he's been here", but then 's stands for "has" and not "is". Again, we cannot have the system approve of 's as a correct contraction of "has", since it is not always true; example: "he's a house".
So, writing in full letters is always preferable on Duolingo!
This is the answer: what you hear at the end of "sucre" is the French schwa.
Further info here: https://frenchcrazy.com/2013/04/the-french-schwa.html/