i said that it was at least ten years since I-----a social dance.

谁说since后面一定要一般式？看情况啦
下面是抄的

since的四种用法

1） since +过去一个时间点（如具体的年、月、日期、钟点、1980, last month, half past six）。例如：

I have been here since 1989. 1989起，我一直在这儿。

2） since +一段时间+ ago。例如：

I have been here since five months ago. 我在这儿，已经有五个月了。

3） since +从句。例如：

Great changes have taken place since you left. 你走后，变化可大了。

Great changes have taken place since we were here. 我们走后，变化可大了。

4） It is +一段时间+ since从句。例如：

It is two years since I became a postgraduate student. 我考上研究生有两年了。

前面用 it had been
since 后面用过去式 was
你后边这句话a danced b had danced
实在看不懂

这句话，在最前面的said提前有了一个过去的时态
所以，后面的is 就变成了was

因为主句是过去时,所以从句就变成它自己的过去,如:一般时变成过去时;现在进行时变成过去进行时;一般将来时变成过去将来时.