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.
