The negative prefix un- attaches chiefly to adjectives,(unable, unclean, unequal, unripe, unsafe) and,participles(分詞) used as adjectives (unfeeling, unflinching, unfinished, unsaid), and less frequently to nouns (unbelief, unconcern, unrest).
Sometimes the noun form of an adjective with the un- prefix has the prefix in-, as in inability, inequality, injustice, and instability.A few stems appear with both prefixes with distinctions of meaning. Inhuman means "brutal, monstrous," while unhuman means "not of human form, superhuman."
When used with adjectives, un- often has a sense distinct from that of non-. Non- picks out the set of things that are not in the category denoted by the stem to which it is attached. un- picks out properties unlike those of the typical examples of the category. Thus nonmilitary personnel are those who are not members of the military, whereas someone who is unmilitary is unlike a typical soldier in dress, habits, or attitudes.
un-主要接形容詞來使成否定型，un-Adj，或者接分詞un-Ving, un-P.P 來當作形容詞。un- 比較少接名詞。un比較常用於字首字尾拆掉後還是一個完整的詞，而in比較多用於接字根。如果是un-Adj，有時候會有相對應的in-N ，例如 unequal --> inequality.