your trousers are dirty,you must have them washed是否正确

Your trousers are dirty. You must have them washed. or
Your trousers are dirty and you must have them washed.

两个独立的句子之间不可用逗号.

本该做什么，但实际没有做应该是：Your trousers are dirty, you should have them washed。吧~~~

我想，这样是不对的，
两个独立的句子之前最好用and连接，

Your trousers are dirty, so why not having them washed?
Your trousers are dirty, and you must have them washed.
Your trousers are dirty, so you must have them washed.

正确
这里是表示一种虚拟语气，即“本该做什么，但实际没有做”
该句话的意思是“你的裤子脏了，你本该洗一洗的”

right