我用几个含有直接引语的句子作一对照，仔细体会。括号略作说明。然后我把网友的句子也放进去，与句6作比较。

1. "I like reading adventure stories," said John. （引语的句末使用逗号，said John 后用句号结束句子）

2. "Surely," said John, "he used to sleep here." （一个句子拆成两部分，said John后用逗号）

3. "Go on with your work," says John. "I'll wait for you till it's finished." （引语是两个独立的句子，says John 后用句号。）

4. "Go on with your work,"  says John,  "and I'll wait for you till it's finished." （引语是and 连接的并列句，says John放在第一分句之后、and 之前，用逗号）

5. “Tom can’t come,”  says John,  “because he is ill.” （says John 在引语的主句和从句之间，says John后用逗号）

6.“I think you can polish this article,”  said John,  as he helped Mary prepare for her writing.（said John 后面也是从句但不是引语，用逗号。5、6可以比较。）

我把句6的said John 放在句首，顺序改为John said：
John said, "I think you can polish this article,” as he helped Mary prepare for her writing.

与网友的句子对比：

I say, "Lucky! Get my book," and he does it at once.
这两句的标点完全一样，直接引语结束时使用了逗号，而不是句号。原因是，后面还一个分句。前者是as 引导的时间状语从句，后者是and 引导的并列句。也就是说：

如果直接引语后面，还有一个与主句有关的从句或并列句，那么，直接引语部分也不能用句号。否则就是标点错误：

John said, "I think you can polish this article.” as he helped Mary prepare for her writing. （误）

I say, "Lucky! Get my book."  and he does it at once.（误）