Private Sub Command1_Click()
Dim x As Integer, y As Integer, comma As Integer
Dim str As String, a As String, b As String
comma = InStr(Text1.Text, ",")
If comma = 0 Then comma = InStr(Text1.Text, "，")
If comma = 0 Then MsgBox "wrong input": Exit Sub
a = Left(Text1.Text, comma - 1)
b = Mid(Text1.Text, comma + 1)
If IsNumeric(a) = False Or IsNumeric(b) = False Then
MsgBox "must be vaild number!"
Exit Sub
Else
x = Val(a): y = Val(b)
End If
If (x > 0 And y > 0) Then
str = "(" & Text1.Text & ")" & "是在第一象限“"
ElseIf (x < 0 And y > 0) Then
str = "(" & Text1.Text & ")" & "是在第一象限“"
End If

End Sub

看起来没啥大问题，
最后的提示错了，另外少了二组。

还有，X＝0 或Y ＝0 时也要提示。这个在坐标轴上。

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If IsNumeric(a) = False Or IsNumeric(b) = False Then 改为
If  IsNumeric(a)  and IsNumeric(b)  Then
IsNumeric(a) 如果为数值，那么返回 真
真 and 真 ＝真
真 and 假 ＝假
假 and 假 ＝假
有一个为假时，说明数据输入有误。