Commas are used after introductory words, clauses, or phrases to set them apart from the main sentence.

逗号也可以用在引导词、从句和短语之中，来把它们和主句区分。

We can use a fall-rise after introductory words and in the middle of a sentence to signal that there's more to come.

我们可以利用在介绍性的词语后面加上降升调，然后在句子的中间表示后面还有话要说。

The handwriting, the seal, the paper, the introductory words, all had told on the instant that the letter had come to him from the hands of the widow Jethway, now dead and cold.

笔迹、印章、纸张、介绍性文字，都在一瞬间告诉他，这封信是从寡妇杰思薇的手中交给他的，她现在已经死了，已经冰冷。