The adverb 实在 (shízài) means "really" or "truly" and is used to emphasize that something is genuinely the case. It carries a tone of sincerity or inevitability, often implying the speaker has no choice or is being completely honest. For example, 实在没想到 means "really didn't expect."
今天实在太热了,我不想出门。
Jīntiān shízài tài rè le, wǒ bù xiǎng chūmén.
It's really too hot today; I don't want to go out.
这道题我实在不会做,你能帮我吗?
Zhè dào tí wǒ shízài bú huì zuò, nǐ néng bāng wǒ ma?
I really can't solve this problem; can you help me?
她唱得实在太好听了,大家都鼓掌了。
Tā chàng de shízài tài hǎotīng le, dàjiā dōu gǔzhǎng le.
She sang so beautifully that everyone applauded.