所谓的祈使句,是指用来表示命令、请求或号召等的句子,其肯定式结构为：动词原形+……
即：Listen to the teacher carefully.委婉一点的话,可以在句首或句末加please,句首需要大写Please,Please listen to the teacher carefully；放在句末主句后需要加逗号,Listen to the teacher carefully,please.