Angular: Adding an import to app.module.ts throws an error

Question

I thought that if you add an import to the controller.ts file, that it is supposed to be added to app.module.ts as well. But, when I added the NgForm to my controller alone without adding it to app.module.ts my code worked fine. Then when I added it to app.module.ts I got an error. What is the right way to do it?