biết đấy mà phải ngậm tăm 
to know it but to be forced to keep silence [ biết ] : to know, know how to do (something) be aware of
[ đấy ] : there, at that place; that
[ mà ] : but, which, where, that, and
[ phải ] : (1) correct, right (opposite of left), right (opposite of wrong); (2) must, have to
[ ngậm ] : to hold, keep; to bear, endure; to close (mouth, lips)
[ tăm ] : bubble, trace, toothpick
Most Common Compounds Set 1:
Most Common Compounds Set 2 :