he has not sent me his address .He does not want me to write to him (so that)

问题描述:

he has not sent me his address .He does not want me to write to him (so that)
连接以上句子,用括号中的词语替代动词want
He has not sent me his address so that I will/shall not be able to write to him.
为什么多了be able to这个短语?
应该是这样呀:
He has not sent me his address so that I will/may not write to him.
答案是不是错了?如果我是错的,请分析一下原因,

你没错,答案也没错那答案为什么多了be able to呢?他不让我写信给他,不就是我不能写信给他么??