"We are slaves to nothing but the clock," it has been said.
这是一整句话。引号里的是直接引语。用逗号，因为句子还没有完全结束。如果把说话人放在前面，就用句号了。It has been said, "We are slaves to nothing but the clock."
说话人在后，前面引语可以用逗号，感叹号，还可以用问号，就是不能用句号。因为句子没结束。
"Did you go to school yesterday?" asked Tom.
"What a day!" he shouted.