"I will watch a movie tomorrow."
"Ashita" isn't an adjective, so it can't just modify "eiga". Two nouns would usually need a particle between them to connect them.
If you see a time period like "ashita" at the beginning of the sentence without a particle, it's telling you when something will happen or happened.
Why is " 映画を明日見ます" marked wrong? An adverb right before a verb is a pattern I've seen in previous lessons. Are there just particular rules for time expressions? It would help if I could find the relevant tips /grammar lesson through Duolingo, but I haven't seen one about this (yet?).
Adverbs of time usually go at the beginning of the sentence. It's not wrong to put it somewhere else, but it changes the nuance. Because your answer is not the standard, that's probably why it hasn't been added to the alternate answers, but I think it would be added if you submit an error report.
This site has a lot of explanation about sentence structure and has pictures of usual word orders: https://8020japanese.com/japanese-adverbs/
So in Japanese, the topic is stated first in the sentence if it is stated at all. In this case, we're basically saying, "concerning tomorrow, ..."
I think that depending on context は (topic marker) seems to be omitted after time expressions and replaced e.g with a comma. Combining the two, we get this situation. Don't be discouraged! Word order is one of the common hurdles that English speakers have with Japanese.
を tells you the direct object of the sentence (who or what receives the action of the verb).
I will watch what? I will watch a movie. 映画を見ます (eiga o mimasu).
I will eat what? I will eat sushi. 寿司を食べます (sushi o tabemasu).
There are some exceptions. You can use は in place of を sometimes to change the emphasis of the sentence. Also, some Japanese verbs don't take direct objects when their English equivalents would.
This sentence could be 明日 without a particle, or it could be 明日は (ashita wa), but に would not be appropriate here.
You don’t usually need the particle に ( = ni) with the followings time words because they already function as an adverb.
Note: There are cases that you use に(=ni). I will explain when to use it later.
(X = wrong ○= correct)
Ex. I will be back tomorrow.
X 明日にまた、来ます。 (wrong)
= Ashita ni mata kimasu.
= Ashita, mata kimasu.
She explains some exceptions in the article, but it's rare to have a context where you would use 明日に.
No, it wouldn't work. If you check the previous comments there's more information: https://forum.duolingo.com/comment/23200459?comment_id=33177045