（ ）you have understand this rule,you have no further difficu

首先 挑个错,you have understand 是没有的东西,应该是 you have understood. (完成式)
或者就用现在式,you understand.
答案选A, Once you have understood this rule, you have no further difficulty.
(你一旦明白了这个规则) (就没有更多问题/困难了.)
其他几个选项要用的话,要嘛改句子,要嘛意思就变样了：
unless=除非...否则
unless you have understood this rule, you have no further difficulty.
除非你明白这个规则,否则是不会有问题的 (规则懂得越多问题就越多?!)
only=只有、唯有
only WHEN you have understood this rule will you (对,这个顺序) have no further difficulty
唯有当你了解...的时候,才不会有更多的难题 (意思对了,但太复杂了,有点彆扭)
until=直到...(的时候)
until you have understood this rule, you will have no further difficulty.
一直到你明白了这条规则为止 都不会有更多难题 (所以大家最好不要明白... (^_^|||)
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老实说 在下觉得这整个句子都有点怪...
一般要不是说 once you have understood.you will have. ( =完成式 後面跟未来式)
你一旦了解"了以後" 就"不会" 再有问题了
要不就是 once you understand.you have. ( =全部现在式)
你一旦了解就没问题了
再不然就是: once you understand.you will have. ( =前半现在,後半还是一样用未来式)
第一个和最後一个都比较顺.
我讲了一堆,好像有点混乱.总之希望对你有些帮助、欢迎更多的问题! (^_^ )