Перевод - Шведский-Английский - "Jag hatar när det händer!"

Hello Lilian
I have voted positive..but I still wonder if you
can't say just: "I hate it when it happens."

Isn't "that" A more definite form than "it" ?
Maybe a stupid question ...

Good morning, Pia!

"that" is a kind of "neutral" word very often used in English, but a few times with a specific meaning. I really don't know how to explain that in Swedish, sorry.

Your sentence is also correct, but I think an English speaker would use "that" instead of the second "it".
It's like in expressions : we can say "It's terrible!", but we will probably say: "That's terrible!"

I see ... I think that you explained it very well, I understand now, thanks!

Have a good day.