Basic derivative, understanding a question.

Hey,

The derivative of $\displaystyle t e^3 $ is $\displaystyle e^3 $. I do not understand why. The answer in my text says that the t is a constant. However, I do not understand how to tell when something is a constant, and when something is not.

Originally in this problem I used the chain rule, and found the solution to be

$\displaystyle (1)(e^3) + (t)(e^3) $

$\displaystyle = e^3 + t e^3 $

which is not the answer.

I am looking for an explanation, in plain english please.

Thank you!