1. Explain it simply
究竟 intensifies a question. It shows the speaker really wants a clear, final answer.
Use it together with a question word such as 什么, 谁, 哪儿, 为什么 or 怎么.
Word order: Subject + 究竟 + question word + ...？ 究竟 comes before the verb phrase.
究竟 is a little more formal than 到底, but they mean the same thing.
2. Show examples
你究竟想吃什么？
Nǐ jiūjìng xiǎng chī shénme?
What on earth do you want to eat?
究竟 before the verb, with 什么
他究竟去哪儿了？
Tā jiūjìng qù nǎr le?
Where on earth did he go?
presses for a clear answer
这究竟是谁的手机？
Zhè jiūjìng shì shéi de shǒujī?
Whose phone is this exactly?
used with the question word 谁
3. Share a tip
究竟 needs a question word - pair it with 什么, 谁, 哪儿, 为什么 or 怎么.