Source: 方有毅老師

破折號的使用
（1）change 用破折號來表示語氣或思想的突變。
He staggered, picked himself up, then fell backward－stone drunk at his wedding reception.

（2） emphasize 用破折號來分隔或強調一插入語：同位語 appositives、例子、定義。

1.同位語：
Two of the strongest animals in the jungle are vegetarians—the elephant and the gorilla.

2.例子：（裡面多個名詞已經用了逗號）
The care-takers—those who are helpers, nurturers, teachers, mothers—are still systematically devalued.

3.定義：
Personal space—“elbow room”—is a vital commodity for the human animal, and one that cannot be ignored without risking serious trouble.

（3）summarize 用破折號來引入一摘要 summary。
She wants high-style clothes, an elegant apartment, a modern car—in short, she wants money.

（4）strengthen 用破折號來加強作者的論點。
These five passages have not been picked out not because they are especially bad—I could have quoted far worse if I have chosen—but because they illustrate various of the mental vices from which we now suffer.