英文的標點符號是配合「主從分明」、「重點在前」的原則來標記。
對英語而言,最自然的順序為先講主要事件,再講附屬條件。因此若是主要子句在前,附屬子句在後,符合正常順序,便不用加上逗號:
I decided to buy the book because it is insightful.
I would buy the book if it helps to overturn misconceptions.
但若是附屬子句在前,主要子句在後,這是反常順序,就必須使用逗號來特別標記,提示句子重點在後:
Because the book is insightful, I decided to buy it.
If the book helps overturn misconceptions, I would buy it.