How can I translate a point to a cell in the datagrid

If the point, say pt, is in screen coordinates, you can use code such as

Point p1 = dataGrid1.PointToClient(pt);
MessageBox.Show(dataGrid1.HitTest(p1).ToString());

If you are using context menus for catching right-clicks in the grid, you have to do a little work to remember where the original right-click point was as the point passed in through the menu event arguments may not reflect this original click. One solution is to remember the click in the grid MouseDown event, and then use the code above to retrieve the grid cell from within the menu handler.

Point rightMouseDownPoint;

private void dataGrid1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
    if (e.Button == MouseButtons.Right)
        rightMouseDownPoint = Cursor.Position;
}

private void menuItem4_Click(object sender, System.EventArgs e)
{
    Point pt = dataGrid1.PointToClient(rightMouseDownPoint);
    MessageBox.Show(dataGrid1.HitTest(pt).ToString());
}