How can i change the focus to be on Form1 from being on the new Form ?

I have a button click wich i click on and it open a new Form.

Then i can click on the new Form listBox and everything.

Now when i want to back to the Form1 and click there on the menu or a button i need to click on it twice since the control/focus was on the new opened Form.

What's the best way to bring back the focus/control to Form1 without clicking on it first ?

Maybe to use an event that if i move the mouse over Form1 area it will bring the focus/control to the Form1 ?

This is how im doing to open/show the new Form:

if (toolStripComboBox2.SelectedIndex == -1 && toolStripComboBox1.SelectedIndex == -1)
            {
            }
            else
            {
                Lightnings_Extractor.Lightnings_Mode lightningsmode1 = new Lightnings_Extractor.Lightnings_Mode(this);
                lightningsmode1.Show();
            }

I want to make that even if the new Form is open without closing it im moving back to Form1 so the control/focus will be on Form1. And also if im closing/exiting the new Form so the control/focus will be on Form1 automatic without the need to click on it area first.

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danieli