They sang happily until it got dark.=They didn't -----------

我来给你细细解释这个句子
They sang happily until it got dark.
untill的前半句多用否定句,当用肯定句时,像这句
他们唱的很开心,偏偏天黑了.
天黑前,他们一直很开心的唱,天黑坏了他们的好事.
注意这里不能用till代替until,意思不一样的.
用作否定句时：not...untill...直到...才
They didn't stop singing happily until it got dark.
知道天黑,他们都不停的唱着.