【求助】manhattan sc guid8 chapter12 助动词用法求解

phoebevann

P202
1. 讲到助动词可以代替较长的verb phrase, 用来省略，举了一个例子：
错误：Our cars were designed to inspire envy, and they ARE.
正确：Our cars were designed to inspire envy, and they DID. 原因：did=did inspire envy
但是我想问为什么不能理解为省略they are designed to ...所以用are呢？