请问All you do now is to complete the form .中的 do 为什么是动词原形

请问All you do now is to complete the form .中的 do 为什么是动词原形 All you need to do is ask the question 为什么 is 后可直接加动词原形 All you need to do is (to) fill in this form.中to为什么可有可无?是什么语 用动词的适当形式填空 （ ）you （ ）anything interesting now 动词原形是do Be后怎么跟动词原形 All we have to do is clean or peel it 请问:Do you have to leave now?中,have to是情态动词吗? 请问这句话的except后为什么用动词原形?He will do anything to help you except try to do sth.中的do可以是be 动词原形吗? 例句：All you have to do is (to) press the button.若主语有个实意动词do用作 all we can do is to 为什么能用两个动词 should不是加动词原形吗?为什么You should learn to do呢? do you now （when the show is） 为什么不能是when the show is