when he got back he d----- that someone had taken his suitca

discovered,发现
mistake,by mistake,固定搭配,错误的
再问： that little tool makes using computer so e----.
再答： easily
再问： 为啥不是easy
再答： use computer so easily,easily 这个副词是修饰动词 use的
再问： 哦 some computer users have started c---- it a cat
再答： 请先采纳，如有问题可再向我求助。