A lot of adjectives can be made into their opposite form by the addition of a prefix (e.g., un,- in-, dis-, il-) to the beginning of the word. The most common prefix is un-. Adjectives which end with -ful (thoughtful, useful, etc) are an exception: they are made into opposites by replacing -ful with the suffix -less (thoughtless, useless, etc) Building skills for the TOEFL IBT 1 ... - tailieumienphi.vn