why ActionResult method parameter is null in asp.net MVC?

I'm trying to send a form to ActionResult method but it is null always.
In fact, I got the error Value cannot be null. but I don't know why I got it the error.

Here are ActionResult code, my view, and model.

public class VocabularyController : Controller
{
private VocabContext _context;
public VocabularyController()
{
_context = new VocabContext();
}
// GET: Vocabulary
[Route("New")]
public ActionResult New()
{
return View();
}
[HttpPost]
public ActionResult Save(Vocabulary word)
{
if (ModelState.IsValid)
{
_context.Vocabularies.Add(word);
_context.SaveChanges();
}
return RedirectToAction("dashboard", "Home");
}
}
==============================
@model EnglishTest.Models.Vocabulary
@{
ViewBag.Title = "New";
}
class="row">
class="col-lg-12">
class="element-wrapper">
class="element-header">New Word Form
class="element-box">
@using (Html.BeginForm("Save", "Vocabulary", FormMethod.Post))
{
class="form-group">
@Html.LabelFor(m => m.Word)
@Html.TextAreaFor(m => m.Word, new { @class = "form-control", @placeholder = "Word" })
@Html.ValidationMessageFor(m => m.Word)

class="row">

class="col-sm-12">

class="form-group">

@Html.LabelFor(m => m.Defination)

@Html.TextAreaFor(m => m.Defination, new { @class = "form-control", @placeholder = "Definition" })

@Html.ValidationMessageFor(m => m.Defination)