The problem was that he didn't know what would hurt her and he didn’t ___ ask her.

1） dare用作实义动词,此时其后的动词不定式可带to也可不带to,且dare有人称和数以及时态的变化.例如：
I dare to jump down from the top of the wall. 我敢从那墙头上跳下来.
She doesn\'t dare (to) meet her teacher\'s eyes. 她不敢与老师对视.
2） dare用作情态动词,后跟动词原形,主要用于疑问句、否定句和条件句中.例如：
How dare she do things like that to me? 她怎么敢对我做那种事?
-Dare you catch the mouse? 你敢去抓那只老鼠吗?
-I daren\'t do that. 我不敢抓.
If you dare say that to our teacher, I would vote for you. 如果你敢向我们的老师说那件事,我就投你一票.拜托，好好回答问题，不要从哪里截了一段了事