With 2 textboxes on your form named TextBox1 and Textbox2;
Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
Dim stringArray() As String = {"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"}
Dim inputNumber As Integer = 0
Dim outputString As String = String.Empty
If IsNumeric(TextBox1.Text) Then
For Each item As String In TextBox1.Text.ToCharArray
inputNumber = CInt(item)
outputString = outputString + stringArray(inputNumber)
Next
End If
TextBox2.Text = outputString
End Sub