The following generates an input element for a model field using a typed helper:
Html.HiddenFor(m => m.FieldName)
The generated field name is FieldName. How do I add a prefix to the name so that it will render as name="prefix.FieldName"?
You can set the prefix for the HtmlHelper with
htmlHelper.ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix = "foo";
So if you set Html.ViewContext.ViewData.TemplateInfo.HtmlFieldPrefix = "Foo" before Html.HiddenFor(m => m.FormId) the resulting field name would become 'Foo.FormId'
I'd recommend writing an extension method for the HtmlHelper rather than handling this logic in the view. You might then want to use the modeltype's name as a prefix.
You can't. You could use editor templates instead. Brad Wilson has a series of blog posts describing them. Scott Gu also covers them in this post.