How to separate view html in multiple files in asp MVC?

Here's one (very simplified) way to do it. You need to use ViewModels.

public class PageViewModel
{
    public string Title { .. }
    public string Content { .. }
    public MyFormViewModel MyForm { .. }

}

public class MyFormViewModel
{
    public string Name { .. }
    public string Address { .. }
    // ... so on
}

Your Main Page View:

@model PageViewModel

<h1>@Model.Title</h1>
<p>@Model.Content</p>

<div class="my-form">
   @Html.Partial("_MyForm", Model.MyForm)
</div>

Your Form Partial ("_MyForm")

@model MyFormViewModel

@using(Html.BeginForm())
{
    <label>Name</label>
    @Html.TextBoxFor(m => m.Name)
}

Your Controller Actions:

// Load Page
[HttpGet]
public ActionResult MyPage()
{
    var viewModel = new PageViewModel();
    viewModel.MyForm = new MyFormViewModel();
    return View(viewModel);
}

// SubmitForm: Accepts MyFormViewModel
[HttpPost]
public ActionResult MyPage(MyFormViewModel viewModel)
{
   // do something with form data
}

On your main page view, you'll notice @Html.Partial(). This loads the partial with a model's property which in this case is MyForm. Due to this, your partial will have an @model of MyFormViewModel.