"他高兴得不知道说什么好" He is so happy that he doesn't know what to say. The grammar here is similar to the "so + adjective + that" form in English. Here, 高兴得= is so happy that After "that" or "得" comes the result. What's the result of him being so happy? The result is that he doesn't know what to say. More examples: 他凶得没人敢和他做朋友。 He is so fierce that nobody dares to make friends with him. 他兴奋得睡不着觉。 He is so excited that he can't sleep. 考试难得大家都不及格。 The exam was so difficult that everyone failed it. 我饿得可以吃下十碗饭。 I'm so hungry that I can finish ten bowls of rice.

这个句子你有什么具体的疑问呢？是句子的意思不明白还是什么别的问题？如果是不明白这句话是什么意思，这句话的具体意思就是：他因为（某事）太高兴，太激动，所以一时间不知道说些什么，来表达他当时的心情。

我不太明白这句话的意思。 请帮帮我. "他高兴得不知道说什么好" He is too happy to know what he should say. In other words, he is very happy and can't express his feeling in words at that moment.