jiějie bìyè nà tiān, fùmǔ jì gāoxìng yòu yǒudiǎnr nánguò.

On the day my sister graduated, our parents were both happy and a bit sad.

Breakdown of jiějie bìyè nà tiān, fùmǔ jì gāoxìng yòu yǒudiǎnr nánguò.

• 高兴gāoxìng: happy
• 那nà: that
• 有点儿yǒudiǎnr: a bit
• 父母fùmǔ: parents

Questions & Answers about jiějie bìyè nà tiān, fùmǔ jì gāoxìng yòu yǒudiǎnr nánguò.

Why is it 姐姐毕业那天 and not 在姐姐毕业那天?

In Chinese, time expressions can directly modify the sentence without a preposition.
姐姐毕业那天 literally means “the day (when) my older sister graduated” and already answers “when?” for the main clause, so 在 is not required.
You can say 在姐姐毕业那天, but it sounds a bit heavier and is less common in everyday speech.

Why is there no 的 in 姐姐毕业那天? Could I say 姐姐毕业的那天?

Both 姐姐毕业那天 and 姐姐毕业的那天 are grammatically correct.

• 姐姐毕业那天 is a bit more compact and colloquial.
• 姐姐毕业的那天 sounds slightly more explicit and emphasized, like “that day when my sister graduated.”

In many cases (especially with time words like 天, 时候), the 的 can be omitted without changing the meaning much.

What exactly does 既…又… mean, and how is it used?

既…又… links two parallel, coexisting qualities, like “both … and …” in English.
In the sentence, 既高兴又有点儿难过 means “(they were) both happy and a bit sad.”

Basic pattern:

• Subject + 既 + Adjective/Verb Phrase 1 + 又 + Adjective/Verb Phrase 2

You can use it with adjectives (既大又漂亮) or verbs (既工作又学习).

Can 既…又… be replaced with 又…又… or 又…也… here?