Why is Yii2 not finding classes?

Question

Yii

I'm new to yii2 and I'm facing the same problem. BASIC application template
User.php model in the models folder:

namespace app\models;

use Yii;
use yii\base\NotSupportedException;
use yii\behaviors\TimestampBehavior;
use yii\db\ActiveRecord;
use yii\web\IdentityInterface;

class User extends ActiveRecord implements IdentityInterface
{

and the SignupForm.php model:

namespace app\models;


use yii\base\Model;

/**
 * Signup form
 */
class SignupForm extends Model
{
.................
   public function signup()
    {
        ....................
        $user = new User();
        ................

SiteController.php :

public function actionSignup()
    {
        $model = new SignupForm();

We go through the action, we see the form. But when submitting, in general, a wonderful error:

PHP Fatal Error - yii\base\ErrorException
Class '\common\models\User' not found

Other models from the same folder with the same namespaces work fine.
UPD

Answer 1

Case 1
The file is called AppointmentForm.php and the class is Appointment s Form. And they should be named the same
Case 2
Look at the stack trace of the error and look for the place where it occurs