What should a worshiper do if the row next to him is not straight in congregational prayer, such as if the worshipers move slightly away from him, or if one row is ahead of another?

The problem of uneven rows can be addressed by educating and alerting people, explaining the texts that indicate the virtue of straightening the rows, and that it is a cause of harmony and affection, and that not straightening them is a cause of aversion and hatred, as in the Hadith: "You must straighten your rows, or Allah will cause disagreement to appear on your faces." You should align yourself with the person next to you, and if one of them deviates, gently pull him. If he refuses, then the responsibility is his. If people differ in the row, point them to straighten it, and if they refuse, then align with whoever is to the right of the Imam. You should do what you can to straighten the row, and alert the Imam to be diligent in straightening the rows.