knock at敲打某东西，He knocked at the melon to judge if it was fully ripe.knock in to把东西敲（撞）进....，He knocked me in to the water.后面有door时敲门用knock at,knock on都可以后面没有door时用knockHe came in without knocking.他没敲门就进来了I knocked on (at)the door,but there was no answer.我敲了门,但没有回应 望采纳...
