You can query the Checked property of the checkbox - see
CheckBox.Checked Property (System.Windows.Forms) | Microsoft Learn[
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Either wrap the whole thing in a test e.g.
If Not chkBox.Checked Then
If currentTime >= BreakTime1 And currentTime <= BreakTime2 And Counter >= 10 Then
Form1.LblBreak.Visible = True
Form1.lblRun.Visible = False
Form1.lblMStop.Visible = False
End If
End If
or include it in the original e.g.
If Not chkBox.Checked And currentTime >= BreakTime1 And currentTime <= BreakTime2 And Counter >= 10 Then
Form1.LblBreak.Visible = True
Form1.lblRun.Visible = False
Form1.lblMStop.Visible = False
End If