在什么情况下加助动词 为什么要加助动词

1.疑问句要加:What do you do on Sunday?
Do you know the price of this phone?
2.表示强调:Tom does have regard fpr others.
Do be patient!
3.一般疑问句的回答：Yes,I do.
至于为什么要加,我想这个问题还是不知道答案更好.