对于关系代词你可以这么理当宾语不在的时候,代词就是用来代替宾语的.比如“I don't know what you are talking about.”如果用关系副词的话就是“I don't know it which you are talking about.”但没人会这么说,所以就有了直接用what代替it which的用法.
对于你举的例句I ever stayed at last year?我读不通.而对于你的问题,加不加介词只取决于他是不是及物动词.比如上面我举的例句中,talking about里的talk就是不及物动词,所以加介词about.而再比如“I saw what you were hitting.”里,hit是及物动词,所以不需要加介词.
希望帮到你!