知人知面不知心

one may know a person for a long time without understanding his true nature (idiom)

to know / to be aware

person; people / CL:個|个[ge4],位[wei4]

face / side / surface / aspect / top / classifier for objects with flat surfaces such as drums, mirrors, flags etc

variant of 麵|面[mian4]

flour / noodles / (of food) soft (not crunchy) / (slang) (of a person) ineffectual / spineless

no; not so / (bound form) not; un-

not to know / unaware / unknowingly / fig. not to admit (defeat, hardships, tiredness etc)

caring / intimate

heart / mind / intention / center / core / CL:顆|颗[ke1],個|个[ge4]