A term used in transformational grammar, referring to a rule which involves moving a noun phrase out of the predicate of a complement sentence. Sentences such as The ball was easy for John to catch were said to be derived by tough movement from the structure NP[it S[for John to catch the ball]S ]NP was easy, via a rule which extraposes the complement (it was easy for John to catch the ball). The rule of tough movement took the non-subject noun phrase from the extraposed complement (i.e. the ball) and substituted it for the initial subject pronoun of the sentence as a whole (i.e. it).
轉換語法術語，指將一個名詞短語從一個補語句子的謂語中移出的規則。像The ball was easy for John to catch“此球易為約翰所接”這類句子被認為是通過tough移位從結構[it [for John to catch the ball] ]was easy派生而來，即通過一條補語外置的規則（it was easy for John to catch the ball“要約翰接此球很容易”）。tough移位規則將外置補語中的非主語名詞短語（即the ball“此球”）移出并用來替換整個句子開頭的主語代名詞（即it）。