When do you use u substitution in integration

When you look at the integrand, you will use u-substitution when the integral contains some function and its derivative. This will make the integration easier.
Also you have the option to change the limits of integration which is always recommended.

Suppose int() is the integration symbol and g(x)dx is the derivative of f(x). U sub will do this:

If you have any questions feel free to ask.