To lie in / consist in (在于)

Structure Abstract subject (关键 · 问题 · 意义) + 在于 + the essence (phrase or clause)

在于 (zài yú) is a literary copula that pinpoints the ESSENCE, the crux or the cause of something: "lies in", "consists in". The subject is usually abstract (关键, 问题, 意义, 原因, 目的) and what follows 在于 can be a noun or a whole clause: 问题的关键在于执行 "the crux of the problem lies in execution". It is the written, abstract counterpart of locative 在 (which places things in space): here there is no physical place, only the point where what matters "resides". Common in essays, analysis and formal speech.

Examples

问wèn题tí的de关guān键jiàn在zài于yú落luò实shí，而ér非fēi口kǒu号hào。
The crux of the problem lies in implementation, not in slogans.
成chéng功gōng的de意yì义yì在zài于yú过guò程chéng，而ér不bù只zhǐ是shì结jié果guǒ。
The meaning of success lies in the process, not only in the result.
两liǎng国guó分fēn歧qí的de根gēn源yuán在zài于yú历lì史shǐ遗yí留liú问wèn题tí。
The root of the two countries' disagreements lies in historical legacy issues.

Levels are indicative: they follow the standard Chinese Grammar Wiki (AllSet Learning) progression, not an official HSK 3.0 mapping. We never assign levels by eye.

This point and the other 370, with audio on every example, inside Wanyu.