英语短文填空中的一个句子的分析：宾语从句,时间状语从句...
You should say “Thank you” when someone ______ you the salt onthe table,when someone walking ahead of you keeps the door open for you,when someone says your work is well done or you have bought a nice thing,or your city is beautiful.这一段中的第一个词应该填一个什么样的词（动词三单?动名词?）怎样判断的?如果是填give,那是填gives,or giving?为什么