be only to be done和be only done有什么区别呀。例句是欧路上的Practical wisdom is only to be learned in the school of experience.
is only learned 和 is only to be learned 语法上都正确，但第一个 is 是表示被动语态的 be（passive be），第二个 is 是准情态动词 be（quasi-modal be），所以含义略有不同。本例中 is only to be learned 等于 can only be learned。
quasi-modal be 详细介绍请看 The Cambridge Grammar of the English Language 206页。
另外，我觉得 “* Practical wisdom is only to be learned in the school of experience.” 作为例句不合格，前后都不通顺，是 gain wisdom，不是 learn wisdom 啊。。与其写“复杂”的中式英语，不如简单地写成 “Wisdom is only to be acquired through experience.”
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多谢大佬指点！辛苦你了