Yī wèi hǎo lǎoshī dǒngdé yánchuánshēnjiào de zhòngyàoxìng.
A good teacher understands the importance of teaching by word and by example.
Tips
usage
言传 = conveying through words, 身教 = teaching through personal action. The idiom praises teachers and parents whose conduct matches their words — and quietly critiques those whose don't.