另当别论

dòu nǐ mèi mei wán shì yī huí shì dàn dǎ tā nà jiù lìng dāng bié lùn le

逗你妹妹玩是一回事，但打她那就另当别论了。

It's one thing to tease your sister, but it's another to hit her.

rú guǒ tā shì yīn wèi shēng bìng quē xí nà jiù lìng dāng bié lùn

如果她是因为生病缺席，那就另当别论。

If she were absent because of illness, it would be a different matter.