Dim connstr As String
        Dim providername1 As SqlClient.SqlConnection
        connstr = "Data Source=WWW-585271DDAED;Initial Catalog=systemdate;Persist Security Info=True;User ID=sa;Password=123456"
        providername1 = New System.Data.SqlClient.SqlConnection(connstr)
        providername1.Open()
        Dim sqlcmd As SqlClient.SqlCommand
        Dim sqlcmdstr As String
        sqlcmdstr = "select * form denlu where username ='" & UsernameTextBox.Text & "' and password ='" & PasswordTextBox.Text & "'"
        sqlcmd = New SqlClient.SqlCommand(sqlcmdstr, providername1)
        Dim rd As SqlClient.SqlDataReader
       rd = sqlcmd.ExecuteReader  ‘怎么提示这里错误啊～～
        If rd.Read() Then
            Dim form1 As New Form1
            form1.Show()
            Me.Hide()
        Else
            MessageBox.Show("帐号密码错误！", "错误提示！", MessageBoxButtons.OK, MessageBoxIcon.Error)
            UsernameTextBox.Text = ""
            PasswordTextBox.Text = ""
            UsernameTextBox.Focus()
        End If

少了一个括号，
rd = sqlcmd.ExecuteReader()

问题解决了吗？
如果没有，就把错误贴上来，大家可以参考一下

看不懂啊  怎么办啊  

还是不行啊~~~

你的SQL语句错了
应该把"form"改成"from"

select * from denlu where usernamet='

from 跟 form 不要搞混淆了，建议去看看sql语句