I have seen this sentence 待てよ おい 今から行ったんじゃ時間が…, can someone tell me what the 行ったんじゃ means? and describe me how the grammar works, or by any chance, provide me some source, where this exact usage is explained?
It's a casual speaking version of Aのでは、B
- Verb plain form/いadj（〜い） + のでは/んじゃ
- なadj（〜な）/ Noun + なのでは/なんじゃ
If A, B
Under those circumstances (A), B
B includes expressions that express a negative attitude, such as "I'm in trouble" and "I will be having a hard time." etc.
I can't take the weekend off, can I? when I'm this busy with work.
It can't be helped if it's raining. Let's go out tomorrow.
今から行ったんじゃ means "If we go there now,..."
You can use a verb plain form which means, you can use either present tense or past tense. たform is often used in Japanese conditional forms. You can think the meaning is the same in most cases regardless of the tense of a verb. 今から行ったんじゃ、and 今から行くんじゃ、 I would translate to “If I go there now,…” for both of them.
For some specific verbs, there is a slight nuance difference, such as “じゅんびする (to prepare)”.
If I start preparing now, I won’t make it.
If I prepare now (subtly implies that "even if I finish preparation), I won’t make it.