原句 I have learned English for about two years.
改为被动句后English变为主语,所以意思是英语被我学了两年,用英文就是
English has been learned by me for about two years.
在这里面,用has因为english是第三人称单数,原句动词learn的被动式是is learned/learnt,因为原句用完成时,改的句子也应该用完成时,所以用"has been learned/learnt".
然后题目只有三个空的话,就把"by me"舍掉,变成
English has been learned/learnt for about two years.