一字之师 - Chinese Idiom

一字之师:

Literally means:

a one-word mentor

Actually means:

a teacher who corrects just one but very crucial mistake

An ancient anecdote goes that someone was credited a one-word mentor for correcting a mistake in a master's verse or for pointing out an ill-considered word so as to make the literature piece more wonderful.

Today it is used to praise someone for proposing a minor change or a suggestion, often appreciative.

古代有过这样的趣事，即有人纠正名家诗文中某个错字，或指出某字用得不妥，使文字更加精彩，因而被赞。

现在用来指因提出一个小小改动或一个小小的建议而受到称赞。具有褒义色彩。