秘而不宣 keep secret; hush sth. up; keep sth. under one's hat; keep sth. dark; keep sth. under cover [under wraps]; keep the news (of sth.) from getting out; not to let anyone into a secret
述而不作 pass on the ancient culture without adding anything new to it; only elaborate the theories of the predecessors and do not have original ideas of one's own; pass on the ancient culture and not to create