"Lupul mănâncă oaia la cină."

Translation: The wolf eats the sheep for dinner.

[zj456]

why "la" and not "pentru"?

[potestasity]

There's no way to express this, other than "because!" :)

Unfortunately, prepositions aren't always used in the same contexts trans-languages. So, even though, "for" could be translated to "pentru" in a lot of contexts (e.g. I would die for you. - Aș muri pentru tine), it could also be translated as "la" (e.g. I had an apple for lunch. - Am mâncat un măr la prânz), or "de" (e.g. I have nothing to give you for Christmas. - Nu am ce să-ți dau de Crăciun).