2个英语知识点1 buy的现在分词是…2 the key to (do/doing)sth

the key to do sth .the key to doing.哪个结构是正确的? 七年级上册英语句型例： like doing/to do sth buy sb sth=buy sth for sb 求英语知识点,如practice doing sth want to do sth be worth doing sth里的doing sth 是现在分词么那是做什么成分 英语语法问题key的用法It's a key to doing sth It's a key to do sth 哪个对 the key to do和the key to doing的意思和区别 造英语句子最好是初三的水平1.allow sb to do sth（最少2个,用主动语态)2.allow doing s the key to doing还是do? 2个英语语法造句,have fun doing sth和take care not to do sth, help,open的过去分词是什么?是before sb do sth.还是before sb doing sth... 求：英语 ……to do sth/doing sth 的所有句形 英语句型中什么时候用do sth,to do sth,doing sth或 to doing sth