当你想表达 "Peter is not as good at Engpsh as Tom" 时，你可以使用 "Peter is not as good at Engpsh as Tom." 这个句子来表达这个意思。

这个句子的来源是英语中的比较级形式，在这个句子中，我们使用了 "as good" 来表示比较级。"Not as good" 的意思是 "不如"。

其他一些例句包括：

She is not as tall as her sister. (她不如她的姐姐高。)
He is not as smart as his brother. (他不如他的兄弟聪明。)
The movie was not as good as the book. (电影不如书好看。)

希望这些信息对你有帮助。