API在VB中声明以后如何使用？

API函数声明以后要怎么使用呢？比如说copyFile的函数声明以后，又该如何使用？
Public Declare Function CopyFile Lib "kernel32" Alias "CopyFileA" (ByVal lpExistingFileName As String, ByVal lpNewFileName As String, ByVal bFailIfExists As Long) As Long
请各位高手解答！

Private Declare Function CopyFile Lib "kernel32" Alias "CopyFileA" (ByVal lpExistingFileName As String, ByVal lpNewFileName As String, ByVal bFailIfExists As Long) As Long

Private Sub Form_Load()
CopyFile "c:\1.txt", "d:\1.txt", true
End Sub

lpExistingFileName String，源文件名
lpNewFileName String，目标文件名
bFailIfExists Long，如果设为TRUE（非零），那么一旦目标文件已经存在，则函数调用会失败。否则目标文件被改写