英语：1.the bridge is built of stone.请问这个句子的语法,built 是动词吧,怎么就这样和is在一块了
2.i have trouble in learning English and i'm so worried could you help me with it 请问此句中 in learning English 能不能换成 on learning English
3.Suxy loves it and works hard on the subject.
4.Chinese is not easy for me ,but i like it very much ,i'm going work hard at it.请问3,4两题 works hard 后面的介词为什么不一样,一个是 on 一个是at?期待你的回答