英語中,省略系動詞通常是在做后置定語的時候,將原先的定語從句的表語變成后置定語或插入語,從而使句子簡單.省略系動詞的同時必須也省略主語（關(guān)系代詞）.例如 The man who is in black hat is my father. 變成 The man in black hat is my father. (in black hat作后置定語). 或 The man, in black hat, is my father. (in black hat作插入語)
至于什么成分充當(dāng)表語無所謂.但不定式作后置定語的情況確實很少見,原因是不定式也很少作定語從句里的表語.下面這句話雖然沒有語法錯誤,但很少這么說： The man who is to solve the problem is my father. 也可變成 The man to solve the problem is my father. 由于不定式有表即將做某事的意思,常用的說法是加一個going,使之成為將來時,The man who is going to solve the problem is my father. 此時一般不省略who is.