SC知識點：（語意明確性）「雙向修飾」（夾心修飾）
很多時候，雙向修飾是一個我們看例句可以懂，但是考試的時候卻不太清楚怎麼判斷的概念。這裡，Dustin要來解釋一下雙向修飾的狀況。
雙向修飾是屬於「歧義」考點，意指修飾語可能會修飾到前面的成分，也可能會修飾到後面的成分，造成句意有雙重解讀。比如：
Unlike the shuttle and earlier spacecraft, with sufficient enough power in fuel cells and batteries for their short flights, a permanently orbiting space station will have to generate its own electricity.
這裡的句意可能被解讀成「太空船（spacecraft）有足夠的能量」（往前修飾），也可能被解讀成「永久環繞的太空站（a permanently orbiting space station有足夠的能量」（往後修飾）
但是實戰的時候，要怎樣判斷真的有雙向修飾呢？雙向修飾要滿足兩個先決條件：
第一個先決條件：會有夾心歧義（雙向修飾）的成分（片語、分詞或子句），一定是文法上可以往前或往後修飾的。
比如：S+V+O, Ving/Vpp/介系詞/when或if等從屬連接詞子句/同位語, S+V+O…這些成分都有一個特色：可以往前也可以往後。那哪些是只有一個方向的呢？像which, who這種。他只能往前。自然就不會有雙向的問題。
比如：
Unlike a typical automobile loan, which requires a fifteen- to twenty-percent down payment, a lease-loan does not require the buyer to make an initial deposit on the new vehicle.
這裡的which根本無法往後，所以沒有雙向問題。
第二個先決條件是：往前或往後都有他能修飾的對象
Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are now drawing solid conclusions about how the human brain grows and how babies acquire language.
Ving往前可以修飾到N，但是往後卻沒有緊接著的邏輯主詞讓Ving修飾（Ving若要往後修飾只有一種情況：Ving, S+V）。這種時候也不會有雙向修飾的問題。
再看這個句子：
The yield per acre of coffee berries varies enormously, because a single tree, depending on its size and on climate and altitude, is able to produce enough berries to make between one and twelve pounds of dried beans a year.
depending可以修飾到a single tree（Ving可以往前修飾名詞），但是要往後修飾只有一種狀況：Ving, S+V，但後面卻沒有緊接邏輯主詞，所以不會往後修飾。
事實上，有一個更簡便的方法可以判斷選項是否為雙向修飾。你可以用手把前後兩個部份分別遮起來，幫助判斷。
舉例而言：
Unlike the shuttle and earlier spacecraft, with sufficient enough power in fuel cells and batteries for their short flights, a permanently orbiting space station will have to generate its own electricity.
遮前面會變成：
with sufficient enough power in fuel cells and batteries for their short flights], a permanently orbiting space station will have to generate its own electricity.
發現可以修飾到後面。
遮後面會變成：
Unlike the shuttle and earlier spacecraft, with sufficient enough power in fuel cells and batteries for their short flights
再舉另一個例句：
Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are now drawing solid conclusions about how the human brain grows and how babies acquire language.
遮前面：
having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood, are now drawing solid conclusions about how the human brain grows and how babies acquire language.
後面沒有邏輯主詞，無法修飾。
遮後面：
Neuroscientists, having amassed a wealth of knowledge over the past twenty years about the brain and its development from birth to adulthood,
最後再舉：
Unlike a typical automobile loan, which requires a fifteen- to twenty-percent down payment, a lease-loan does not require the buyer to make an initial deposit on the new vehicle.
遮前面：
which requires a fifteen- to twenty-percent down payment, a lease-loan does not require the buyer to make an initial deposit on the new vehicle.
which無法往後修飾。
遮後面：Unlike a typical automobile loan, which requires a fifteen- to twenty-percent down payment,
可以修飾前面的名詞loan。所以不算雙向修飾。
p.s. 雙向修飾不是絕對的錯，只是歧義。有些正確答案還是會有雙向修飾。（歧義考點請放在文法跟合理性之後判斷）
About Dustin
最不專業的GMAT教學分享平台。
最討厭商學院卻考了GMAT。
為GMAT考生打造的輔助學習網站。
（那個，拜託，不要叫我老師或顧問。）