once upon a time是一个固定搭配短语，释义：从前；很久以前
ago 是一个单词，既是形容词也是副词，和 "long'搭配使用。
once upon a time 很久很久以前
例句：Once upon a timein a faraway land there lived a princess in a big castle.
从前，在很远的地方，有一位美丽的公主住在一座很大的城堡里.
Once upon a timea big, fat frog lived in a tiny shallow pond.
从前,有一只小的, 胖的青蛙住在又小又浅的池塘里.
3.
Once upon a time, there lived three monks in a small temple.
从前, 一个小寺庙里住着三个和尚.
Once upon a time there lived a king who had an ugly daughter．从前有一位国王，他有一个丑陋的女儿。
ago adv.以前adj.以前的n.
例句：The person you were looking for died three yearsago.你找的那个人三年前就已经去世了。
A long timeago, the man was arrested for drug trafficking.很久以前，这个男人就因为贩毒而被逮捕
