Is is used when you are talking about a singular object for example "the house is empty", while are is used when you are talking about more than one object for example "the houses are empty"
I'm not very good at writing the rules, but I can give you some examples. Hopefully somebody can improve this answer later on. Here are my examples. I do. He does. They do. You do. It does.
You can use the chain rule to find the derivative of the composite of two functions--the derivative of the "outside" function multiplied by the derivative of the "inside" function. The chain rule is...