You should say "Thank you"when someone passes you the salt o

You should say "Thank you"when someone passes you the salt on the table,when someone walking before
you keep the door open for you,when someone says you have done your work well,you have bought a nice thing,or your city is very beautiful.（1~2）
1.为什么when someone says 中用says而不用saying,况且上文也有when someone walking before you keep the door open for you
（难道是句中的before 可是都是介词后的动词加ing呀）
2.为什么when someone walking before you keep the door open for you中要用keep而不用keeping,句子中不是有before吗
3.Have your parents flown to Shanghai?
Yes.They went there to see the fashion show.
问句中为什么用现在完成时,答句中用过去时.