They have been to Beijing only once.对only once提问

请问they have been able to do的时态是什么?been able to 和 be able to的区别是什么? They have been going to Arcades where computer games can be played for a small cost. They found the lecture hard . 　　A. to be understood B. to understand 　　C. for understandingD to have been understood have been done 和 to be done [W43] When I went to the Smiths',they happened _______ dinner.A.to have B.to be having C.to have been having D.having 翻译,并分析. --What _ to them?--they appeared__in the accident.A.has happened to,be injured B.happened ,to have been injuredC.has happened to ,be injured D.happened ,injured 关于英语主动变被动的小问题.They have to built many houses since last year.变为被动后：Houses have to been built since last year.这么变对么?be 为什么要改为 been?没有be 变成 been 谁帮我翻译一下Maybe for those that have been missed,they are fated to be this way. Where have they been? to be done to have been done to have been doing 形式的造句, They have never been to hong kong改为肯定句 they have been to Beijing_twice_.(划线部分提问) They have already been to Sandy Bay改成疑问句 They have never been to Bei Jing 怎么改反意疑问句? They have been to America.(改为一般疑问句) They have been to Tiannamen Square 改为一般疑问句 i think they have been to england的反义疑问句? They have been to the sydney.有语法错误吗