is being 和are being

is being 和 are being 都是被动语态的正在进行时态 。
is 和 are的使用取决与主语的人称数：一般情况下，第三人称单数就用 is ；you 及复数就用 are 了：
eg：
You/they are being told how to dance by the teacher at the moment (now).
She is being told how to dance by the teacher at the moment (now). 举了同样的例子便是为了于对比区别。
下面的句子是正确的：I could dance very well when I was still young..
英语只要掌握基本的语法知识，多读就可以学的很不错的（个人意见）

i could dance very well when i was still young..
这句对
is being 和are being 主语单复数不同
过去式could后面不用pat tense 吗？ 没看懂

这句话是虚拟句