that is (to say) used to give more exact information about something or to correct a statement 亦即，那就是说，换句话说：One solution would be to change the shape of the screen, that is, to make it wider. 有一个方法，就是改变屏幕的形状，或者说，把屏幕变得宽一些。/ Languages are taught by the direct method, that is to say, without using the student's own language.语言教学使用的是直接教学法，也就是说，不用学生的母语。/ I loved him - that is, I thought I did. 我当时是爱他的 — 或者说，我以为我是爱他的。（摘自《朗文高级英语应试词典》pp.2465-2466）

    从上面的实例可以看出，that is 或者 that is to say 通常放在句子中间，它的前面应该用逗号或者破折号，后面应该用逗号。