grammar - 「～たじゃない」 expression in spoken Japanese

I noticed in an anime I watched, one of the characters said something like below:

さっき食べたじゃない。

And what I think the meaning is: Didn't you just eat a few while ago?

From what I have learned in Japanese classes, 「じゃない」, which is the shortened form of 「ではない」, must follow a noun word/phrase/clause. But in the spoken sentence above the 「じゃない」 follows a 「～た」 form of a verb. Is that sentence grammatically correct? Is this one of the many examples where colloquial language may skip a few grammar rules here and there?

I think the sentence would be grammatically correct if there is　「ん」 in between the two phrases: 「さっき食べたんじゃない」. Did I actually mishear?

Answer

I do not know if さっき食べたじゃない is grammatically correct or not, but sentence + じゃない is a common colloquial construct whose meaning is similar to a tag question as Amanda wrote: “You ate it a little while ago, didn’t you?” It is a statement rather than a question, and the じゃない part indicates either that the speaker is surprised by the fact that he/she has to say it or that the speaker wants a confirmation.

An example of surprise:

A: 冷蔵庫に入れておいたケーキがない! (I cannot find the cake I put in the fridge!)
B: あんたさっき食べたじゃない。 (You ate it a little while ago, didn’t you.)

An example of wanting confirmation:

A: さっきケーキを食べたじゃない。あれじつは1週間前に賞味期限が切れていたの。 (We (or you or …) ate cake a little while ago, didn’t we? To tell the truth, that cake was best before a week ago.)

I think that さっき食べたんじゃない has a different meaning from さっき食べたじゃない. さっき食べたんじゃない has two meanings:

1. Without the raise of pitch at the end of the sentence, it is a negation of さっき食べた. “It is not true that I ate it a little while ago.” The usual negation of さっき食べた is さっき食べなかった (I did not eat it a little while ago), and they have different meanings, but I cannot explain it clearly.


2. With the raise of pitch at the end of the sentence (often denoted by a question mark: さっき食べたんじゃない?), it is a question “Didn’t you eat it a little while ago?” with indication that the speaker thinks that “you” probably ate it a little while ago.