Why do you leave out the の in some cases?

Question

An example of this is that I have seen both 来週テスト and 来週のテスト. Both of them mean "next week's test".

My issue is that I have no clue when to use which or what makes them different from each other. In what situations can you leave out the の?

Edit: Just to clarify, the example I've mentioned above is one of many instances I've heard, as in when listening to shows to better grasp spoken Japanese (opposed to textbook). I'm trying to get a general guideline as to situations when one leaves it in, and when one takes it out.

Answer 1

Here is the critical difference:

• 来週の always modifies a noun (aka "adjectival").
• 来週 always modifies a verb (aka "adverbial").

来週, 明日, 昨日 and so on do not take any particle when used adverbially. (In English, words like "tomorrow" do not take prepositions when used as an adverb, either. For example, you say "I'll do it tomorrow" rather than "I'll do it on tomorrow".)

テスト is a noun, so you should always use 来週のテスト to say "next week's exam".

On the other hand, 来週テスト is simply an incorrect phrase at least in formal Japanese. In colloquial Japanese, you can understand this as an abbreviated sentence where がある is omitted (note that ある is a verb), i.e., it can be an abbreviated sentence meaning "I have an exam next week". Either way, it never means "next week's exam".

Note that Japanese is a language that strictly distinguishes noun-modifying expressions and verb-modifying expressions. See also: using の with と,で, から, まで

Answer 2

The meaning "来週テスト" is often used as a casual or abbreviated meaning among the general Japanese population, often in the sense of "来週テストがある"The correct grammar is "来週 、　テストがある" or "来週 はテストがある"

来週のテスト means (a test that is next week). Or next week's test

This is very difficult for even native speakers to understand sometimes.

Answer 3

It's determined by the will of the speaking human. There are no rules and no guidelines. There is no need in specification, because it is used the semantics of kanji to understand the sense of the phrase. It's passed in the official documents, written speech, newspaper articles etc.