The modal adverbs 究竟 and 毕竟
The modal adverb (语气副词) expresses subjective judgment or assurance toward an action.
究竟 
and 毕竟 can be used to emphasize a characteristic or a reason for something.
and 毕竟 can be used to emphasize a characteristic or a reason for something.
Examples
妈妈究竟是妈妈，她多么累也不会给我们看
Mother is mother, she won't show us how tired she is
māma jiùjìng shì māma, tā duōme lèi yě bú huì gěi wǒmen kàn
他毕竟还小，别骂他
Stop yelling at him. He's only a kid, after all
tā bìjìng hái xiǎo, bié mà tā
Pay attention
If the predicate is expressed by a noun, the verb 是 is placed after 究竟 and 毕竟.
If the predicate is expressed by an adjective, 是 is not placed after 究竟 and 毕竟.
is placed after 究竟 and 毕竟.
If the predicate is expressed by an adjective, 是 is not placed after 究竟 and 毕竟.毕竟 is often used with the words 虽然 , 无论 and 不论 .
, 无论 and 不论 .Examples
他不论是什么样的爸爸，他毕竟是你爸
Whoever he may be, he is still your father
tā búlùn shì shénmeyàng de bàba, tā bìjìng shì nǐ bà
他们无论多么有钱，毕竟都是人
No matter how rich they are, they are still human
tāmen wúlùn duōme yǒuqián, bìjìng dōu shì rén
究竟 is also used in interrogative sentences to get an answer from someone.
Pay attention
毕竟 cannot be used in these sentences.
Examples
究竟是谁想出那个笑话的？
Who came up with that joke?
jiùjìng shì shéi xiǎngchū nà ge xiàohuà de
她究竟去不去？
Is she going or not?
tā jiùjìng qù bú qù
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