1. He speaks as a teacher.
句中的 as 表示身份,句意为:他对老师的身份讲话。
2. He speaks like a teacher.
句中的 like 表示相似,句意为:他说起话来像老师。
两举一组例句供参考:
He speaks as an expert. He is after all a professor. 他以专家身份讲话。他毕竟是位教授嘛。
He talks like an expert, but really he knows very little. 他说起话来像个专家,但实际上他所知甚少。