否定式是在實(shí)義動(dòng)詞think前加助動(dòng)詞do,然后助動(dòng)詞按照時(shí)態(tài)變位并采用否定式（用否定副詞not修飾do或者do的變位）,think變成動(dòng)詞原形think,如：
She doesn't think she is beautiful.
應(yīng)該注意的是,樓主給出的句子是一句復(fù)雜句,也就是句子中的某個(gè)成分是一個(gè)從句,在這里從句是she is beautiful做think 的賓語.
帶賓語從句的句子,即使賓語從句想要用否定式,通常也要在主句上用否定來實(shí)現(xiàn).比如：
She is beatiful.這是一句簡(jiǎn)單句,它的否定式是She is not beautiful.
把這句話作為賓語從句用到She thinks主句中時(shí)就是She thinks she is beautiful.
但是,如果要把she is not beautiful用在主句中的話,那么not就要移到主句里變成She doesn't think she is beautiful.
這是通常的表達(dá)習(xí)慣.說的簡(jiǎn)單點(diǎn),也就是不管主句還是賓語從句,只要用到了否定,那么否定副詞都放在主句動(dòng)詞上.