在工程硕士GCT英语考试中，情态动词是历年的考察重点，如果考生牢固掌握GCT英语情态动词的用法，可以大大提高英语成绩，如下北京在职研究生招生信息网提供的情态动词对比解析，一起来了解下吧!
情态动词+ have +过去分词
1) may(might) have + done sth, can (could) have + done sth 表示过去，推测过去时间里可能发生的事情。
Philip can (could) have been hurt seriously in the car accident.
2) must have +done sth,对过去时间里可能发生的事情的推测，语气较强，具有“肯定”,“谅必”的意思。
-Linda has gone to work, but her bicycle is still here.
-She must have gone by bus.
3) ought to have done sth, should
本应该做某事，而事实上并没有做。否定句表示“不该做某事而做了”。
You ought to (should) have been more careful in this experiment.
He ought not to have thrown the old clothes away.(事实上已扔了。)
ought to 在语气上比should 要强。
4) neednt have done sth 本没必要做某事
I dressed very warmly for the trip, but I needn?t have done so. The weather was hot.
5) would like to have done sth 本打算做某事
I would like to have read the article, but I was very busy then.
比较have to和must
1) 两词都是‘必须’的意思，have to 表示客观的需要，must 表示说话人主观上的看法，既主观上的必要。
My brother was very ill, so I had to call the doctor in the middle of the night. 我弟弟病得很厉害，我只得半夜里把医生请来。(客观上需要做这件事)
He said that they must work hard. 他说他们必须努力工作。(主观上要做这件事)
2) have to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。
He had to look after his sister yesterday.
3) 在否定结构中： don?t have to 表示“不必” ;mustnt 表示“禁止”。
You dont have to tell him about it. 你不一定要把此事告诉他。
You mustnt tell him about it. 你一定不要把这件事告诉他。
通过以上对比，相信各位考生对于GCT英语情态动词的用法有了更为条理的认知。其实GCT英语语法等同于数学公式，符合语境进行简单套用即可。