Do you know what events are? No?
Do you know what a newsletter email is? Ok, good, that's an event.
SubscribeEvent is how you tell the newsletter organizer that you want to receive the email.
Now you know how events work.
If you are registering a class object to the event bus your event handers must be static. To have non-static handlers working register an instance of your class to the bus.
https://mcforge.readthedocs.io/en/latest/events/intro/#static-event-handlers
This is based on your resources pack format. If you have a pack.mcmeta file that specifies the pack_format as 3 then all resource names MUST be lowercase including the lang file. If you are using format 2(happens if you do not specify the format at all then forge will use LegacyV2Adapter) then lang files must be as en_US.
How are you downloading the modpack?
If you are using a pack from CurseForge using the Twitch launcher it handles Forge differently and installs it for you.
Note: A Forge "Installation" is not like your typical Windows program installation, it is paired with an instance or profile of Minecraft and is dependent on its location relative to said instance or profile..