Solved Random errors while using IntelliJ?

Okay. Here's the deal. I switched to IntelliJ IDEA and created a new test project to see if I liked it. I really liked it and so I was planning on switching to it. However, I kept getting these random errors when I knew the code was correct, I've used all of these statements in Eclipse so was wondering why I was getting the error. Here's an image: https://gyazo.com/7c836b09fb479c92a2ba88709559ccbb Go full screen to see it easily. Please leave why this is happening below! I really would like to switch to IntelliJ but can't because of these errors

 

@timtower Well, When I try to export the file it's coming up with:
==================================
Error16, 30) java: cannot find symbol
symbol: class CommandSender
location: class CorePlugin.Core
==================================
Error16, 52) java: cannot find symbol
symbol: class Command
location: class CorePlugin.Core
==================================
Error13, 66) java: cannot find symbol
symbol: class chatHandler
location: class CorePlugin.Core
==================================
Error29, 29) java: cannot find symbol
symbol: variable p
location: class CorePlugin.Core
==================================
Error32, 25) java: cannot find symbol
symbol: variable p
location: class CorePlugin.Core
==================================

Using = to separate errors.

 

@Mindlessmink
You haven't imported the CommandSender and Command classes. If you put your cursor where you use these classes and press Alt + Enter, you get a list of imports to pick from.

 

Ohh, I never knew how to import. I assumed it was ctrl + shift + o (same as Eclipse)

 

@Mindlessmink
CTRL + ALT + O it is in intellij

and you can activate
File -> Settings -> Editor -> General -> Auto import and tick "Optimize imports on the fly"
to let intellij handle that automatically.