"The shoe is red."
Translation:La chaussure est rouge.
If you enter "le chaussure", which is wrong because "la chaussure" is a feminine noun, the system will propose the next masculine synonym, which is "le soulier".
By the way, "une chaussure" and "un soulier" are not really interchangeable, since in France, the noun "un soulier" is almost never used (but it is in Quebec).
As already explained, when you use the wrong gender, the system keeps the wrong article and suggests a synonym matching this gender.
So, if you wrote "un chaussure", which is wrong, because "une chaussure" is feminine, the system will suggest "un soulier", which is masculine.
I am not sure I have understood your suggestion.
The very first time you see a new noun on this course, it has an article (or determiner of some kind).
Not only does it help you remember that French nouns use articles in 99% of cases, but it also teaches you which gender the noun is.
If you learn: "apple = [une pomme] as if the article were a prefix, you have a better chance to remember the noun's gender next time you need it.
That really is a great idea. However, Duolingo's program is set to process the words of the sentence in a linear fashion. So if it sees "le", it looks among all the possible answers to find a sentence that has "le" in that position. The only one is "le soulier". I'm afraid that Duo's staff would be loathe to scrap the entire program to re-write it in the way you describe.