Добавить цикл строки текстового поля

Я не уверен, что вы пытаетесь достичь с помощью своего кода, но я сделал несколько комментариев, которые могут помочь.

Private Sub DoSomething()
        Dim words() = {"Mathew", "Mark", "Luke", "John"}
        Dim found As Boolean
        For Each word In words
            'How about checking if the number of lines in TextBox1 is
            'equal to or more than the the number of lines in TextBox2.
            'If it is less than the code will fail with an index out of range error
            If TextBox1.Lines.Count < TextBox2.Lines.Count Then
                MessageBox.Show("Sorry TextBox1 does not have enough lines.")
                Exit Sub
            End If
            For i As Integer = 0 To TextBox2.Lines.Count - 1
                TxtbValBeforeCompar.Text = TextBox1.Lines(i)
                CompareNumbers()
                If TextBox1.Lines(i).Contains(word) Then
                    found = True
                    'What is the point of tempTextBox? If is never visible and falls
                    'out of scope at the end of the Sub.
                    'Check that the number of CheckVars is not less than the number
                    'of lines in TextBox2
                    'REMEMBER The first value of i is 0 so you need a CheckVar0 TextBox
                    Dim counter As Integer
                    For Each ctrl As Control In Controls
                        If ctrl.Name.StartsWith("CheckVar") Then
                            counter += 1
                        End If
                    Next
                    'Of course you 
                    If counter < TextBox2.Lines.Count Then
                        MessageBox.Show("Sorry, not enough CheckVarX controls.")
                        Exit Sub
                    End If
                    'What is the point of tempTextBox? If is never visible and falls
                    'out of scope at the end of the Sub.
                    Dim tempTextBox As TextBox = CType(Me.Controls("CheckVar" & i.ToString), TextBox)
                    'You could include the nested If in the outer If
                    'with AndAlso
                    If TextBox2.Lines(i).Contains(word) Then
                        'Use the Not keyword an get rid of the empty If
                        If Not tempTextBox.Text.Contains(word) Then
                            tempTextBox.Text = tempTextBox.Text + " " + TxtbValAfterCompar.Text()
                        End If
                    End If
                End If
            Next
        Next
    End Sub
    Private Sub CompareNumbers()
        'unknown code
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        DoSomething()
    End Sub