>>405
Not having got any expertise in teaching Japanese, I can't assure my thought is right but here is an answer of mine.
To begin with, a noun followed by "は" doesn't make a significant part of a sentence. It (almost) always makes only a complement.
Therefore, the backbone of the sentence would remain intact even if you eliminated the noun followed by "は" from the sentence.
Then, "宿題は明日します" arguably is a reply to a question like "When will you do your homework?"
If so, "Tomorrow." or "明日します." is a sufficient answer to the question.
However, "明日宿題をします" probably is a reply to a question like "What are you going to do tomorrow?"
To answer this question, in contrast, a noun followed by "を" could never be eliminated from the sentence because it is itself the backbone of the sentence.
As a result, in turn, an adverb "明日" turns into a complement of the sentence.
Your reply this time will be like "I'm gonna do my homework." or "宿題をします."
All of my knowledge is based on a thin paperback:『日本語という外国語』（荒川洋平、講談社現代新書）