completion. Compare final.
comprehend. Compare apprehend.
conciseness. Compare brevity.
conclude should not be used for "close." To conclude is a mental process; to close a physical one.
condign means "well-merited "; therefore, the common phrase "condign punishment" is correct, but the phrase "Deserving (or not deserving) condign punishment," is absurd because tautological.
conduct: Although the dictionaries give both a transitive and intransitive place to this verb in the signification of "behave," it should properly be used only reflexively, as a transitive. Say, "How did the débutante conduct herself?" rather than "How did the débutante conduct?"
confess. Compare own.
congratulate. Compare felicitate.
congregation, corps: Exercise care in the use of these words. A congregation is an assemblage of persons who meet as for religious worship or instruction; a corps is a body of men associated in some specific work, as a marine corps; a corps of engineers. A congregation embraces both sexes, corps is restricted to the male sex.
con man: A vulgar term for a swindler's decoy or "bunco-steerer"; a confidence man: not used in polite society.