"I came to you at night."
Translation:Przyszłam do ciebie w nocy.
Is there a reason why I can't use the short form, cię, here, or has it just not been added yet? Also, I haven't quite figured out when to use the short and long forms (ciebie, cię; jego, go; etc.), so if someone could explain it or send a link to some site offering an explanation on it, that would be very kind.
Well, the longer ones are accented, so often they are acceptable, but not that natural. For example, only an accented form can go at the beginning of a sentence.
Also, only an accented form can be used after a preposition. So "do cię" can't work.
And some of them have a separate form for using after a preposition. "to him" will be "do niego", neither "do go" or "do jego" will work (well, "do jego X" can be "to his X").
"przychodzić" is the imperfective verb, so it would work here if the sentence was "I always used to come to you at night". See the conjugation here: https://en.wiktionary.org/wiki/przychodzi%C4%87#Conjugation
What we have here is the perfective "przyjść". See the conjugation here: https://en.wiktionary.org/wiki/przyj%C5%9B%C4%87#Conjugation