最近有讀者在後臺提到了一個問題：
這裡提到的句子出自The Economist在6月份的一篇文章Have Siberian fires been smouldering underground all winter?
Seen from the sky, the northern stretches of Siberia in early May were a splodgy white, their thinning winter snow cover interrupted by the brown veins of meandering rivers. Nestled within some of these curves, though, satellites picked up patches of soil warmer than the ground around them. The patches grew and multiplied as the month went on. Before it was over, some were visibly ablaze.
段落意思是：從天上望下去，5月初的西伯利亞北部地區是一片斑駁的白色，覆蓋在上面的冬季積雪在不斷變薄，被蜿蜒的褐色河流切開。但是衛星發現一些處於河流拐角處的土壤比周圍的土地要更加溫暖。這些土壤在5月份面積不斷增大，到5月結束之前，一些土壤已經明顯燃燒起來了。
這裡值得討論的句子是第二句：
Nestled within some of these curves, though, satellites picked up patches of soil warmer than the ground around them.Nestled within some of these curves這一分詞結構在句子中充當狀語，其中nestle的用法是something is nestled among/within XX，含義是「某事物坐落於XX之間」，比如《韋氏高階英語詞典》裡面就有這樣的例句：The vacation resort was nestled among the hills. 那個度假勝地坐落在群山之間。
根據語境，nestled within some of these curves這一部分修飾的是patches of soil，即：patches of soil were nestled within some of these curves，但句子的主語卻是satellites，也就是說，句子中做狀語的分詞短語的邏輯主語跟句子的主語不一致，句子事實上出現了「垂懸結構」（Dangling modifier）。
所謂的垂懸結構是指非謂語動詞短語和介詞短語作狀語時，其邏輯主語與主句主語不一致，導致出現了邏輯問題。垂懸結構是一種很常見的寫作問題，不僅英語學習者很容易犯，就連英語母語者也經常會出錯。比如《衛報》就曾寫過一篇文章提醒讀者要注意寫作中的垂懸結構問題：
具體來講，垂懸結構一般出現句首，包括分詞、不定式以及介詞短語等，比如下面這幾個例子：
(1) Turning the corner, a handsome school building appeared.
(2) To improve his results, the experiment was done again.
(3) At the age of eight, my family finally bought a dog.
第一個句子中作者想表達的意思是「轉過拐角後一座雄偉的學校建築出現了」，但turning the corner的邏輯主語應該是描述者，而不是學校建築，因此這裡出現了垂懸結構問題。為了統一主語，這裡可以改為：Turning the corner, he saw a handsome school building.
第二個句子中to improve his results邏輯主語應該是人，而不是實驗本身，句子可以調整為：To improve his results, he did the experiment again. / He improved his results by doing the experiment again.
第三個句子想表達的意思是在主人公8歲時，他家終於買了一條狗，但是英文句子中at the age of eight可以理解為「在我的家庭8歲時」，從而造成表意偏差。我們可以將句子改為：My family finally bought a dog when I was eight years old.
需要注意的一點是，當分詞短語用來表明說話者的態度或意見時，它們不需要邏輯主語，這個時候形成的垂懸結構並不算錯。對此，維基百科關於垂懸結構的詞條裡面有這樣的說明：
Participial modifiers can sometimes be intended to describe the attitude or mood of the speaker, even when the speaker is not part of the sentence. Some such modifiers are standard and are not considered dangling modifiers.這種情況一般出現在常見的幾個結構裡面，比如judging by/from, given (that), considering (that), owing to, provided (that)，比如柯林斯詞典中就有這樣一個例句：Judging from the way he laughed as he told it, it was meant to be humorous.
此時Judging from的邏輯主語和主句主語it並不一致，但Judging from…已經成了一個固定結構，用來表達說話者的態度，相當於「根據……判斷」，因此這一句子也是正確的。
參考資料：
[1] https://en.wikipedia.org/wiki/Dangling_modifier
[2]https://www.theguardian.com/media/mind-your-language/2010/aug/04/dangling-modifiers-hanging-participles
[3]https://owl.purdue.edu/owl/general_writing/mechanics/dangling_modifiers_and_how_to_correct_them.html
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