[VB.NET] Basic VS Access แสดงและเพิ่มข้อมูล ติดปัญหาข้อมูลไม่แสดง

Public Class Form1
    Dim dbProvider As String
    Dim dbSource As String
    Dim con As New OleDb.OleDbConnection
    Dim sql As String = "SELECT * FORM tb1"
    Dim ds As New DataSet
    Dim da As OleDb.OleDbDataAdapter
    Dim i As Integer
    Dim len As Integer

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.Load
        dbProvider = "PROVIDER=Microsoft.jet.OLEDB.4.0;"
        dbSource = "Data Source = C:\student.mdb"
        con.ConnectionString = dbProvider & dbSource
        con.Open()
        da = New OleDb.OleDbDataAdapter(sql, con)
        da.Fill(ds, "student")
        len = da.Fill(ds, "student")
    End Sub

    Private Sub nav()
        TextBox1.Text = ds.Tables("tb1").Rows(i).Item(1)
        TextBox2.Text = ds.Tables("tb1").Rows(i).Item(2)
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        i = 0
        nav()
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If (i = 0) Then
            MsgBox("This is First")
        Else
            i = i - 1
        End If
    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
        If (i = len - 1) Then
            MsgBox("This is Last")
        Else
            i = i + 1
        End If
    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click
        i = len - 1
        nav()
    End Sub


End Class

Private Sub nav()
        TextBox1.Text = ds.Tables("tb1").Rows(i).Item(1)
        TextBox2.Text = ds.Tables("tb1").Rows(i).Item(2)
    End Sub