根据所给词的适当形式填空。
You can’t expect ________ (learn) to drive in such a short time.
Let’ stop ________ (exercise) unhealthily.
The students will have a sports meeting if it ________ ( not rain) tomorrow.
________ you _______ (buy) a cellphone? No, he _______ (buy) one this afternoon.
I ________ (keep) these stamps for three years.