Betty used to be naughty child的同义句

应该是Betty used to be a naughty child.
同义句：Betty once was a naughty child.
Betty used to be naughty.

Betty was a naughty child when he was young。

used to be表示“过去曾经/常常是......”（但是现在不是这样了）。比如；
He used to be shy in front of people.他过去见到人很害羞。（含义是：现在不这样了）
Betty was a naughty child.

其实应该是used to do更合适。
He used to walk along the beach after dinner when he lived in Qingdao.他住在青岛的时候，常在晚饭后到沙滩散步。（现在可能不在青岛，也不去散步了）

另外，注意该结构与 be used to 的区别。这里to 是介词，后面跟名词、动名词作宾语。意思是：习惯于......
I am used to eating pepper.我现在习惯吃辣椒了
Betty used to doing naughty child.