iOS app "next" key won't go to the next text field

The return key doesn't do anything special in a text field by default. You need to explicitly change the first responder:

- (BOOL)textFieldShouldReturn:(UITextField *)theTextField {
    if (theTextField == self.textPassword) {
        [theTextField resignFirstResponder];
    } else if (theTextField == self.textUsername) {
        [self.textPassword becomeFirstResponder];
    }
    return YES;
}

I use this code that allows me to control the form behavior in the storyboard:

-(BOOL) textFieldShouldReturn:(UITextField *)textField{
    if(textField.returnKeyType==UIReturnKeyNext) {
        UIView *next = [[textField superview] viewWithTag:textField.tag+1];
        [next becomeFirstResponder];
    } else if (textField.returnKeyType==UIReturnKeyDone) {
        [textField resignFirstResponder];
    }
    return YES;
} 

All you need is to assign the order value as tag, and returnkey according to Next or Done on each input control.

for those who want to use Swift language:

class MyClass: UIViewController, UITextFieldDelegate {

@IBOutlet weak var text1: UITextField!
@IBOutlet weak var text2: UITextField!
@IBOutlet weak var text3: UITextField!

override func viewDidLoad() {
    super.viewDidLoad()

    text1.delegate = self
    text2.delegate = self
    text3.delegate = self


}

//....

func textFieldShouldReturn(textField: UITextField) -> Bool{
    if textField == self.text1 {
        self.text2.becomeFirstResponder()
    }else if textField == self.text2{
        self.text3.becomeFirstResponder()
    }else{
        self.text1.becomeFirstResponder()
    }
    return true
 }

 //...

}

:-)