時々　「ときどき」　is what I have learnt and seems to be commonly used. There may be other ways of saying it, but I don't know them.

It's true that the direct translation of "sometimes" is "ときどき". But Japanese people (especially young ones I guess) barely say "ときどき" during conversation. We say "たまに [tamani]" to describe the frequency of "sometimes".

“時々” is correct answer. And we also use “たまに”. It means “sometimes” in English as well. and i think it is used more in usual conversation.

Hehe it's a little confusing >< But with your information now I can understand a bit more about it :D Thanks for your help!

As JapaneseSpecialist-san said, 忘れることがあります = He sometimes forgot = There are cases he forgets. I think it's a bit tricky when you translate it.