byは締め切り「～までに」、untilは期間「までずっと」
A by B
A--->B
She stayed by 4 o'clock.（駄目）
She will come by 4 o'clock.　4時までに

A until B
A<--->B
She stayed until 4 o'clock.　4時までずっと
She will come until 4 o'clock.（駄目）

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■例1
Please wait until I come.　はOK
She will come until we get there.　は駄目
She will not come until we get there.　はOK
She will come before we get there.　はOK
She will come by the time we get there.　はOK

by は前置詞なので後ろに節をつける場合は、by the time ---のようにします。
until は前置詞と接続詞の両方があるので、後ろには名詞、節の両方をつけることができる。
byはある時点、untilは継続

■例2
I'll be there until three o'clock.　3時まで居ます
I'll be there by three o'clock.　3時までには到着します

■例3
Postpone until --- はOK、　Postpone by --- 駄目