yī yán nán jìn
cannot fully express in one sentence
it’s a long story; it’s hard to explain in a few words
Qízhōng xì qíng, yīyánnánjìn.
It would take a whole book to give all the details. or It’s impossible to describe in a few words.
说来话长, 其中底细, 一言难尽。
Shuōlái huà cháng, qízhōng dǐxi, yīyánnánjìn.
It would take too long to tell you that in full.