Getting input from text field in ViewController and using in another class

When you do infinitive = MainViewController().input, MainViewController() just creates a new instance of MainViewController. That new instance never calls textFieldShouldReturn and therefore its input will be nil. Instead, add a reference from Conjugate to MainViewController.

var mainViewController: MainViewController?

Then in textFieldShouldReturn create a Conjugate and assign its property:

let conjugate = Conjugate()
conjugate.mainVieController = self

Then in the conjugate() method, instead of creating a new view controller, refer to the property:

func conjugate() {

  // gets verb from text field
  infinitive = self.ainViewController.input

  // checks verb ending and sets value to booleans
  if (infinitive.hasSuffix("ar")) {
      isEndingAr = true
      }
  }
}

Also, like @emresancaktar said, input should be optional, since it might be nil.

You initialize a new MainViewController instance in conjugate() method which is totally a different object than the actual main view controller.

You should figure some way out for passing the textField's text onto Conjugate class:

One way would be:

class Conjugate {
  ...

  conjugateWithController(viewController: MainViewController) {
    if let infinitive = viewController.input {
      ...
    }
  }       
}

and you should pass the actual MainViewController instance to that method instead of creating a local one.

var infinitive: String!
var input: String!

Thats the problem. You did not initiate your class so its nil. You should use the ? for the variable.

try these :

var infinitive: String?
var input: String?