不知……好: not knowing what to do or say
【高80】固定格式 / set structures
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不知 + question word + verb + 好 voices perplexity: you do not know what to do or say. 这件事很复杂，不知如何解决才好。看他紧张得满头大汗，我也不知说什么好。 The 才 before 好 is optional; 好 means “(it would be) right, fitting”: literally, “I don't know how to solve it so that it is right”.
The question word can be 怎么, 如何, 什么, 哪个… In speech you also hear 不知道 (不知道怎么办才好); 不知 on its own is slightly more written, but the pattern is very common in conversation. The sentence is a statement of helplessness, in the first or the third person, and it does not end with 吗 or 呢.
It differs from 不知道 + clause (“I don't know whether, what…”), which merely notes ignorance, and from 不知所措 (“at a loss”), a complete phrase that takes no question word.
Examples
这件事很复杂，不知如何解决才好。
Zhè jiàn shì hěn fùzá, bù zhī rúhé jiějué cái hǎo.This matter is very complicated, and I don't know how best to solve it.
看他紧张得满头大汗，我也不知说什么好。
Kàn tā jǐnzhāng de mǎn tóu dà hàn, wǒ yě bù zhī shuō shénme hǎo.Seeing him so nervous that sweat was pouring down his face, I didn't know what to say either.
面对满桌子的菜，客人们不知先吃哪个好。
Miànduì mǎn zhuōzi de cài, kèrénmen bù zhī xiān chī nǎge hǎo.Faced with a table full of dishes, the guests did not know which to eat first.
孩子哭个不停，妈妈不知怎么哄才好。
Háizi kū ge bù tíng, māma bù zhī zěnme hǒng cái hǎo.The child would not stop crying, and his mother did not know how to soothe him.
钱不够，时间又紧，我真不知怎么办才好。
Qián bú gòu, shíjiān yòu jǐn, wǒ zhēn bù zhī zěnme bàn cái hǎo.There isn't enough money and time is short; I really don't know what to do.
老师当众夸他，他红着脸，不知说什么好。
Lǎoshī dāngzhòng kuā tā, tā hóng zhe liǎn, bù zhī shuō shénme hǎo.When the teacher praised him in front of everyone, he blushed and did not know what to say.
Common mistakes
✗ 我不知好怎么办。 → ✓ 我不知怎么办才好。 (好 closes the pattern).
✗ 不知怎么办好吗？ → ✓ 不知怎么办才好。 (it is a statement of helplessness, not a yes/no question).
✗ 他不知所措怎么办。 → ✓ 他不知所措。 / 他不知怎么办才好。 (不知所措 is complete and takes no question word).