妳提的問題有些亂，整理了壹下，大概包括如下幾個方面的問題。
1、關於it的四種用法暨回答妳的疑問
1）人稱代詞：指代已知的或上文中提到的中性名詞，包括事物、動物和不知性別的人。如：
Polish the table until it shone.把桌子擦出光亮。
It is chasing a cat.它在追壹只貓。
Who is it knocking at the door?是哪位在敲門呀？
It doesn't add to my confusion about your predicament.有關妳的境遇前面提到的那件事並沒有給我增加我任何迷惘。這是妳提到的第壹個問題。句中的it不是形式主語，而是指代上文提到過的壹件事。
2）非人稱代詞：指代時間、天氣、距離、度量衡、情況等。如：
It is Sunday today.今天是星期日。
It was fine that day.那壹天天氣晴朗。
It is not far from here.離這裏不遠。
3）強調代詞：用“it is +強調成分+ that/who”指代要強調的部分。強調句是用來強句子主語、賓語、狀語的。以妳提到的句子為例：
I did notrealize my mistake until then.註意句中用的是過去時態，因此強調句應該用it was … that，
強調主語：It was (I) that/who did not realize my mistake until then.
強調賓語：It was (my mistake) that I did not realize until then.
強調狀語：It was (not until then) that I realized my mistake.由於否定詞not隨同until then壹起提前，原謂語動詞不在需要助動詞did，所以要改用過去式realized
4）先行代詞：用作形式主語或賓語，指代拖後的不定式短語、動名詞短語或從句。如：
It took him some hours to do the task.形式主語，it指代不定式短語to do the task
It is no help going there.去那裏無濟於事。形式主語，it指代動名詞短語going there
I found it important to do something.我覺得做壹些事是很重要的。形式賓語，指代to do something
答疑：It doesn'tadd to my confusion about your predicament中不含不定式、動名詞或主語從句，由此也可以說明本句中的it不是形式主語。
2、關於until的用法
Until表示“壹種行為壹直延續到某壹個時間點為止就反其道而行之”，因此，只有延續的行為才能與until連用。
1）如果謂語動詞具有延續性，那麽這個動詞就可以壹直進行到until所截止的時間，如：
We worked until 10 o'clock.我們壹直工作到10點（就停止工作了）。
The meeting continued until midnight.會議壹直開到半夜（才結束）。
2）如果謂語動詞是壹縱即逝的瞬間動詞，那麽就需要把這個動詞變成不進行的延續狀態，也就是要用否定句表示這種不延續性，如：
He didn't get up until 8 o'clock.（在8點之前他壹直都在進行著不起床得延續行為），直到8點他（反其道而行之）才起床。
I did not realize my mistake until then.（在那個時候之前他壹直都處於沒有意識到我的錯誤的延續狀態），直到那個時候他才意識到了我的錯誤。
3、關於狀語的位置
狀語是壹種附加成分，位置比較順便，不強調時壹般放在句末，強調時要放在句首，如果是順便提及，還可以插入句中。如：
正常語序：One important thing she discovered was that chimps hunt and eat meat. Everyone had thought chimps ate only fruit and nuts (until then)
強調語序：One important thing she discovered was that chimps hunt and eat meat. (Until then) everyone had thought chimps ate only fruit and nuts.
順便提及：One important thing she discovered was that chimps hunt and eat meat. Everyone, (until then,) had thought chimps ate only fruit and nuts.
4、其他
1）Her work changed the way people think about chimps.（= Her work changed the way that people think about chimps其中that是指代the way的關系代詞，由於在從句中作thought的賓語而被省略。單另用壹句話寫就是people think the way about chimps）
2）表示時間的名詞作狀語，如果前面帶有this，that，last，next，the other等限定詞，前面不能用介詞on或者in。如：
I am going to see you this week.
He will leave for Beijing next month.
I came across a book that you might like when I was doing shopping in the market the other day