Not every forcing move is a good one. Black's four on row 7 looks perfect: it demands white's reply. But look at the diagonal white is building — after the forced block lands on 9,7, that very stone completes white's own four, and white's four is an OPEN four: two five points, unanswerable. The tempting move hands white the winning stone. The honest move is 9,7 instead: black's own gap four, and the stone that caps white's three before it was ever a four.
Solve the remaining tasks to finish this lesson.