"The dog gets angry when he sees cats."
Translation:Hunden blir arg när han ser katter.
Why is 'när den ser katter' accepted....that would be 'The dog gets angry when IT sees cats'
Differences between languages. Pets are usually referred as "him/her" in English, where as it's probably more common to use "it" in Swedish, I would guess. I don't know for sure how it is in Swedish, but it varies a lot by language. For example in my native language (Finnish) it would simply be wrong to refer to a dog as "he".
That would work, if the English sentence said it, but it says he so the translation has to match.