How do I separate items use linq?

I understand, you're trying to group the articles by year and then by month.

Let's say you have the following Article class:

class Article
{
    public DateTime PostDate { get; set; }
    public string Title { get; set; }

    public Article(DateTime postDate, string title)
    {
        PostDate = postDate;
        Title = title;
    }
}

And the following sample data:

var list = new List<Article>
{
    new Article(new DateTime(2012, 12, 1), "A"),
    new Article(new DateTime(2012, 12, 1), "B"),
    new Article(new DateTime(2012, 11, 2), "C"),
    new Article(new DateTime(2012, 11, 2), "D"),
    new Article(new DateTime(2013, 12, 1), "E")
};

You could do the grouping like this:

var grouped = list.GroupBy(a => a.PostDate.Year)
              .Select(a => new { Year = a.Key, Items = a.GroupBy(i => i.PostDate.Month)
                                                        .Select(i => new { Month = i.Key, Titles = i.Select(t => t.Title)})
                                                        .OrderBy(i => i.Month)})
              .OrderBy(a => a.Year);

And output it like this:

foreach (var year in grouped)
{
    Console.WriteLine("Year: " + year.Year);
    foreach (var month in year.Items)
    {
        Console.WriteLine("  Month: " + month.Month);
        foreach (var title in month.Titles)
        {
            Console.WriteLine("    Title: " + title);
        }
    }
}

Here's the output:

Year: 2012
  Month: 11
    Title: C
    Title: D
  Month: 12
    Title: A
    Title: B
Year: 2013
  Month: 12
    Title: E