public function actionMyControllerAction()
{
    $this->requirePostRequest();

    // Build a model of the data you are submitting
    $model = new MyModel();
    $model->someOptionalAttribute = Craft::$app->getRequest()->getBodyParam('someOptionalAttribute');

    // The `getRequiredBodyParam` method can help validate required fields
    $model->someRequiredAttribute = Craft::$app->getRequest()->getRequiredBodyParam('someRequiredAttribute');

    // After you prepare the model, hand things off to a service
    // to validate and/or save things to the database
    // The "saveThisModel" method can run any additional
    // validation you need to run, add errors to the model,
    // and return `false` if validation fails
    if (!Plugin::getInstance()->myService->saveThisModel($model)) {

        Craft::$app->getSession()->setError(Craft::t('my-plugin', 'Unable to save item.'));

        // This is where you name the thing you are returning to your template with errors.
        // In this example, the 'myModel' variable would be available to your submitted template
        Craft::$app->getUrlManager()->setRouteParams([
            'myModel' => $model
        ]);

        return null;
    }

    Craft::$app->getSession()->setNotice(Craft::t('my-plugin', 'Item saved.'));

    // Redirects to wherever the submitted 'redirect' input points to
    return $this->redirectToPostedUrl();
}

{{ dump(myModel.getErrors()) }}

{{ dump(myModel.getError('someOptionalAttribute')) }}

{{ forms.textField({
    label: "Title"|t('app'),
    siteId: category.siteId,
    id: 'title',
    name: 'title',
    value: category.title,
    errors: category.getErrors('title'),
    required: true,
    maxlength: 255
}) }}

if (!Craft::$app->getElements()->saveElement($category)) {

    // ...

    Craft::$app->getSession()->setError(Craft::t('app', 'Couldn’t save category.'));

    // Send the category back to the template
    Craft::$app->getUrlManager()->setRouteParams([
        'category' => $category
    ]);

    return null;
}