The correct answer is 明日なの, not 明日. Why is that? My reasoning is that テストが明日を would mean "the test is performing an action on tomorrow", but we want to apply を to the whole テストが明日, so we need to turn this phrase into a "noun" by adding の, and because 明日 itself is a noun, な is needed in between.
Is this correct?
Is this correct?
Yeah, basically.
Given
テストが明日を知りませんでした
Then as you say, we get テスト as a subject and 明日 as a direct object, giving us something like
The test didn't know tomorrow (???)
Which is generally not a reasonable thing to say.
With the below:
テストが明日なのを知りませんでした
We have
(I) didn't know that the test is tomorrow
Where the object of verb is the clause テストが明日なの. The most natural subject here is an implied "me", mostly because the only reading of the sentence that makes sense is one in which テストが is part of the subordinate clause.
If having a subordinate clause without an explicit verb makes you uncomfortable, it may help to think of this as テストが明日であるのを知りませんでした, which is mostly equivalent in terms of meaning. である sounds fairly stiff/academic though so なの is much more common.
If you were asked to put what you didn't know in a sentence (in polite style), you would say:
テストは明日です。
It's a sentence with a nominal predicate, meaning it ends with [N]-です. (No action is involved here.) To connect such a sentence to の, you need to add な as [N]-なの.
［テストが明日な］のを知りませんでした。
The same rule applies to a sentence that ends with [な-Adj]-です.
［テストが簡単な］のを知りませんでした。
Verbs and い-adjectives, on the other hand, connect directly to の in their various plain forms.
明日テストがあります。
［明日テストがある］のを知りませんでした。
テストは難しいです。
［テストが難しい］のを知りませんでした。
です works like normal verbs in past tense as its past plain form だった connects directly to の with no connector like な.
テストは昨日でした。
［テストが昨日だった］のを知りませんでした。
