有句台词是：there is rules，rules 是复数形式，be动词为什么要要用is？

有句台词是：there is rules，rules 是复数形式，be动词为什么要要用is？

这是非正式的用法，口语里常见。正规书面用法里应当是 There are rules...
语法网站上有如下解释：
Note that in everyday English, many people use there's with a plural subject:
There's two boys in the garden.
There's two boys and a girl outside.
There's a lot of dogs in the street.
This is increasingly common in British and American English, but it is not recommended in formal situations.

口语里可以这样用。按正规的用法应该是there are 再看看别人怎么说的。