Need help with adding a class to an element only when another element is active

I need to add a class to an element when another element's class is showing active. This is the only way I can figure out how to solve my issue which involves incorrect padding on a sticky menu when the user scrolls down/up. We will need to use jQuery and I am having trouble with this. I believe this should be a simple function. I would like to be shown how to do this in my Wordpress theme and use Teamviewer and Skype for communication. I have SFTP and all the credentials ready.