不知道…好【bù zhīdào … hǎo】

不知道 + question word + verb + 好 says you can't decide what to do – which choice would be the good one. The 好 at the end is "would be best"; it turns plain not-knowing into being at a loss.

The question word can come before the verb (怎么, 什么时候) or after it (说什么, 选哪个, 去哪儿).

才好 in place of 好 makes the helplessness stronger.

With or without 好

Without 好, the sentence is plain fact: you lack the information. With 好, you're stuck choosing.

Where it goes wrong

The question word can't be dropped, and 好 goes last.

• × 我不知道说好 → 我不知道说什么好
• × 我不知道好说什么 → 我不知道说什么好