英语语法问题，急！！！used to在句中充当什么成分

Question

句子“There used to be a tree here."
USED TO充当什么成分？
变反义疑问句时是按照THERE BE句型么？
可以举例句说明么？

Answer 1

1 used to 充当时间状语
2 是
3 eg: BE there used to a tree here

Answer 2

USED TO充当状语
变反义疑问句There used to be a tree here,didn't there?
是按照THERE BE句型

Answer 3

used to 过去经常。。。；过去曾，应当是谓语的一部分
They used to play cards a lot.

Answer 4

used to是曾经。。。的意思，这句话翻译为这里曾经有课树

Answer 5

1. used to 是 时间状语
2.是
3.Was there a tree here?

Answer 6

1 used to 充当时间状语
2 是
3 eg: BE there used to a tree here