1题.首要要清楚,begin作“开始”讲,是“瞬时动词”,即“非延续性动词”,因此,不能用进行时、现在完成时,不选BD.此时的how long ago引导宾语从句,从句中要用陈述句语序,举个例子,I wonder when it began而不是I wonder when did it began/did it began/was it beginning.因此,在AC选项中,排除A选项,选C.错题原因：不知道how long可以引导从句,对从句的引导词不熟.
2题.如果用it seems that 的话,后面是一个从句,从句中没有主语.如果加上个there is,即it seems that there is still some elements undiscovered就对了.
3题.全句是两个表语从句,即you're saying that...,and that....
disagree 与agree 搭配相同,为不及物动词,跟人是用介词with sb.,跟物用to sth.如果选what,就是disagree what,显然不对,却了个介词,如果说成to what的话,至少语法上是正确的.
4题.与3题相比,此处多了个介词at,介词＋关系代词＝关系副词.when/where/why/how为关系副词,相当于介词＋关系代词(what,which等）.请比较：
This is the place where I lived.
= This is the place at which I lived.