在大学英语六级考试中，单独考察英语六级语法的题型几乎是没有的，但是语法知识点却贯穿整张试卷，下面新东方在线英语六级频道为大家整理了大学英语六级基础语法知识，希望可以为大家带来帮助，一起来学习一下吧。
　　 英语六级基础语法知识：情态动词
　　1、情态动词的完成式 1)must have done——表示对过去的事情较有把握 的、肯定性的推测，常译为“一定，准是，肯定”，其否定形式是can’t/couldn’thave done，意为“决不可能，不太可能”。 e.g. Since the ditch is full of water, it must have rained last night.
　　2)should/ought to have done——表示过去应该做的 却没有做，常含有后悔、责备、埋怨的口吻，意为“本应该”，“应该就好了”; e.g. With all the work finished, I should have gone to your birthday party last night.
　　3)shouldn’t/ought not to have done——表示过去不该 做的事却给做了，也常含有责备、不满之意，意为“本不该”。 e.g. You shouldn’t have gone back to work yesterday without the doctor’s permission.
　　4)may/might have done——表示对过去的事情的推 测，但不十分有把握，意为“可能，或许已经”。 e.g. A traffic accident happened yesterday and a driver may have been hurt.
　　5)could have done——表示过去有可能或有能力做某 事，而实际未做、未能实现的事;有时也表示轻度批评，意为“本来可以，完全可以”。 e.g. The gas leakage could have been avoided if the gas company took immediate measures after the warning call.
　　6)needn’t have done——表示过去做了某事，但没有做的必要，意为“其实没有必要”。 e.g. I needn’t have bought all that wine——only three people came. 、可作情态动词用的短语 would rather, would sooner, would (just) as soon, had rather, had better, had sooner, cannot but, may/might as well [例1]You all those calculations! We have a computer to do that sort of thing. A. needn’t have done B. must not have done C. shouldn’t have done D. can’t have done [例2]I to him because he phoned me shortly afterwards. A. ought to have written B. must have written C. couldn’t have written D. needn’t have written [例3]You those letters. Why didn’t you A. should post B. must have posted C. should have posted D. ought to post