I can barely understand it, let alone explain it.我几乎不明白，更别提解释它了。

Let [=(more commonly) leave] your sister alone.别打扰你姐姐。

All I ask is to be let alone. [=(more commonly) left alone]拜托，别打扰我了。

Would you please let it alone?请你别碰它，好吗？

I never did learn to let well enough alone. [=to leave well enough alone; to stop changing something that is already good enough]我从来没学会适可而止。