helping verb omission and comparison

[ZHUOC614]

Hello instructor,

I found these on page 253 of the Manhattan SC guide:

The first instance of the verb should usually match the helping verb in tense. If you need to change
tenses, repeat the whole verb in the new tense.
Wrong: I have never seen an aardvark, but last year my father DID.
Right: I have never seen an aardvark, but last year my father saw one.
In the rare cases in which the tenses do not need to match, the exact verb form missing after the helping
verb should be present elsewhere in the sentence.
Wrong: Our cars were designed to inspire envy, and they ARE.
Right: Our cars were designed to inspire envy, and they DO.


I found these are really great rules, but I really didn't understand what does the rare cases mentioned in the bold face means?

Thanks a ton!!!

[tim]

"Rare cases" means they don't occur often. Is that what you were asking about? If not, I may have misunderstood your question.