1.句子句首不能出現動詞，除了命令句(祈使句) ：祈使句都是動詞原形常句首，因為省略了主詞you

例：(You) Stand up!

例:
Keeping your tires properly inflated is something few people consider and yet it can improve gas mileage by 10 percent or more. Finally,try to drive on straight roads with few stops.
Avoid curved roads , and roads with lots of lights and traffic.

本句 Avoid curved roads , and roads with lots of lights and traffic.中，AVOID就是命令句的用法，(請你)避開彎彎曲曲的路，過多號誌和雍塞的路。(希望你依指示做，就是命令句)

2.

Sit down, please.

Open the door.

Sing a song together!!

Get the phone, will you?

凡是以原形對詞開頭,主詞略去,

而說話的對象指的是你的時候,

就是祈使句或命令句!!

祈使句語氣較弱,命令句語氣較強,

這是就語意而言,就句型來看,

兩者結構是都一樣的!!!