不同語言具有不同的語法結構，所以只用逐字翻譯的做法並不能得到一個流暢的翻譯句。理論上，我們可以從所有可能的詞組排列情形中找出一組最佳的翻譯結果，但由於計算量過於龐大，因此一般翻譯系統會設定限制條件以降低計算複雜度。例如傳統的distortion model (Brown et. al, 1993)會使用“distortion limit”來限制兩個單詞重排之後的距離，但這種做法，並不適合用在語法結構差異大的兩種語言上。因此近年來的研究已逐漸改為利用語法結構來決定詞組的相對位置。在本論文中，我們利用剖析樹的三層語法子結構以及function words來判斷翻譯句中的詞組排列順序，另外我們也利用詞組替換的方式讓翻譯更加流暢。
實驗結果顯示我們的方法對於翻譯流暢度的提升是有效的，並且在我們的語料庫大小規模之下，使用三層語法子結構會比使用四層以上子結構有著更好的效果。

Since the syntactic structures are often different between two languages, we can not translate a sentence fluently without reordering phrases. Theoretically speaking, we may find out the best translation from all possible permutations of phrases, but since the cost of computation might be too much, the conventional translation system will not consider all permutations. For example, the traditional distortion model (Brown et. al, 1993) uses the “distortion limit” to restrict the distance between two target words after translation, but this doesn’t work well on two languages whose syntactic structures are highly different with each other. Thus recent works have gradually used the syntactic structure to decide the order of phrases. We use the 3-level syntactic sub-structures in a parse tree and function words to determine the order of phrases. Also, we substitute some phrases to make the translation more fluent.
Our experimental results show that it is useful to make the translation more fluent by using our methods. Besides, for the scale of our training corpus, adopting the 3-level syntactic sub-structures is better than adopting the higher level ones.

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