can you say this without the "i"?
It's accepted, yes.
What's the difference?
Not much. "Hålla i" has more of a sense of "holding on to"
I got this one wrong, but after realizing my error I remembered studying the phrasal verb "håller i". However, the correction I got was "håller om". Are the two prepositions interchangeable?
"Håller i" is more like holding on to, whereas "håller om" means embrace.
So if you just hold on to that person's hand, you would use håller i, but if you have your arms around that person, you would use håller om.
Is "dig" even in the word bank for this? I didnt see it...
The accusative for "you" is "dig", it is definitely in the word bank.
I said "Jag i haller dig" instead of "haller i dig". D'oh.
Why isnt jag håller ni acceptable
It's an object here so it needs to be in the object form er. In English they happen to be the same for you, but compare with we, you wouldn't say They hold we.
In fact some dialects would alow this but its pretty rare