"We had decided to eat it."
Translation:Vi havde besluttet at spise den.
Ah, I believe (though I don't remember for certain now) that "det" was not accepted. I'll report it the next time I encounter it.
I put det, and it was accepted but counted as a typo. I know we always use det if it is the subject. Is there a rule for other places in the sentence?
Here, it depends on the grammatical gender of what "it" refers to.
In "Vi havde besluttet at spise grøden" (we had decided to eat the porridge) you can replace "grøden" with "den" but not "det" because "grød" is common gender.
In "Vi havde besluttet at spise brødet" (we had decided to eat the bread) you can replace "brødet" with "det" but not "den" because "brød" is neuter.
At least, I believe that covers it. As a native Dane I was never actually taught this technical stuff about gender. We know what sounds right but not always why :)