Translation:I go to work on Monday.
I think in this case the subject of the sentences is "I" as in "I go to work on Monday". If we put は after 月よう日 it would make " Monday" the subject, thus meaning you are talking about monday in the context and not about you. But then again, it all comes to the context you're using. However, I'm still learning so... Not 100% sure
Well... I can offer a suggestion why it may not have accepted (I personally feel that your sentence should have been accepted though).
It may be because the sentence is vague as to whether "work" is a noun or a verb. In Japanese, the difference would create different sentences.
And maybe it was due to that that it wasn't accepted. I would report it if it still doesn't accept.
Edit on the app;
It's because of the word "going". The sentence I gave as 月曜日に仕事に行きます should have read as 月曜日に仕事に行くつもりです. The word "going" implies intent that would be characteristic of つもり. The sentence given in the question is a simple declaritive and not express intent
I apologize, your sentence shouldn't be right in this case.
If you are talking about what you understand from the sentence, then yeah, there isn't really a difference. However, avoid falling into this trap when learning new languages and focus on the exact translation, as that'll let you fill in the blanks when you learn new vocab, or when you have unexpected vocab presented to you.
As a side/technical note: Its quite often I have to go to work to do something or another without actually being on shift and working. Yes, I go to work Monday, but no, I don't work Monday. Or, if you're taking a sick day, yes, I work Monday, but no, I'm not going to work Monday. Food for thought.
Maybe not backwards, but arranged differently. Japanese is SOV (where the subject often goes first, then the object followed by the verb) while English is SVO. Because of that, the sentence structure varies quite a bit ftom English (which sucks since I'm used to thinking in English and speaking Japanese without a coherant English sentence in mind, leading to really weird sentences coming out of my mouth)