be able to 是什么时态

be able to 是什么时态 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? be able to造句短一点,不管哪个时态! be able to do sth 更多用于什么时态? be able to 和 can 分别用于那种时态? can与be able to的时态上的区别 be able to的be是什么词 can 和be able to的时态上的区别是什么?要有例句一定要有例句啊! be able to 否定形式是什么? be able to do sth be able to do sth 的同义词是什么? be able to 用法 be able to中文 able to be assigned be able to be able to (送分拉) 请问：be able to be able to 意思 I wonder if it's because I haven't been able to be outdoors for so longI haven't been 是什么时态 现在完成时被动语态吗?