When their stay in Spain was over, Ove carried Sonja's huge bags to the bus. He noticed that the bus driver smelled of wine, but didn't think much of it. Ove felt the baby move as they drove and then got up to use the bathroom. When he was halfway to the back of the bus, the bus lurched and glass exploded. People screamed and Ove was thrown around. He couldn't find Sonja, and would forever be haunted by the feeling of helplessness he felt in that moment.
During the accident, Ove is unable to do anything. For him, this is the absolute worst situation for a variety of reasons, not least because his principles value doing over talking. Further, in this situation where Ove has no option to follow his principles, his family is harmed.
For the first week after the accident, Ove sat by Sonja's bed and didn't leave until the nurses insisted he shower and change. A doctor told Ove to prepare himself for the worst, and Ove threw the doctor through a closed door. Sonja briefly woke on the tenth day, found Ove's hand, and fell back asleep. When she woke up the following day, Ove insisted on telling her himself that the baby was gone. Sonja cried and grieved and Ove knew he'd never forgive himself for not being able to protect Sonja and the baby.
Ove is exceptionally loyal while Sonja is in the hospital, and he demands that Sonja's healthcare providers act the same way. Notice that both Ove and Sonja take the time to mourn and grieve for the family that, it's implied, they're not going to have now. Ove knows that he's going to carry this grief around with him forever, which explains some of his depression in the present.
Days later, Sonja told Ove she wanted to start physiotherapy. Ove looked at her as though she was crazy, and she told him that they needed to move on. Back in Sweden, Ove met more white shirts, one of whom wanted to place Sonja in assisted living and spoke as though Sonja wasn't in the room. Ove threw that woman out and threw one of Sonja's shoes after her. When he went later to ask the nurses where the shoe went, Sonja laughed and Ove's chest felt looser than it had in weeks.
Unlike Ove, Sonja insists that they move forward and put their grief behind them. There's never an indication that she regrets the trip or even what happened, which suggests that Ove's habit of regretting his inability to help and refusal to forgive himself is one of the most destructive elements of memory and grief. It doesn't allow Ove to truly move on from the trauma.
Ove rebuilt the house and made it accessible for Sonja's wheelchair. She returned to teacher training the day after she left the hospital, finished in the spring, and applied for a job teaching troubled students. She got them all to read Shakespeare.
Sonja uses what happened to move forward and continue doing good in the world. Ove can finally do something productive when he rebuilds the house for Sonja.
Ove remained angry. He wanted to destroy everyone involved in the accident and began writing letters to anyone who might be able to do something. Nobody would accept responsibility. He tried to sue the council for refusing to install an access ramp at Sonja's school. Men in white shirts kept stopping Ove, and Ove never forgave them. Finally, one spring day, Sonja stopped Ove in the middle of writing a letter and told him it was enough. Ove built the ramp at Sonja's school himself, and she came home every day and told him about her students. Sonja said that everyone needs to know what they're fighting for. The narrator says that Sonja fought for her students, and Ove fought for Sonja.
The implication that Ove had nobody to fight for but Sonja, or simply didn't want to fight for anything or anyone else, is shown in the present to have disastrous consequences. While in the flashbacks where Sonja is still alive it gives Ove purpose and an outlet, in the present it leaves Ove completely lost. The bureaucracy, represented by the men in white shirts, means that Ove is never able to translate his own principles and sense of fairness into reparations for Sonja.