Ramon walked slowly around the edge of the park. He wondered where everyone was. It was a sunny, beautiful day - the kind where he would have expected his friends to be outside playing. If it had rained, he understood the lack of people. Then, he finally spotted his friends and was very happy. "Come over and play baseball!" they yelled.

Which sentence has an incorrect form of the mood?

He wondered where everyone was.
It was a sunny, beautiful day – the kind where he would have expected his friends to be outside playing.
If it had rained, he understood the lack of people.
"Come over and play baseball!" they yelled.

"If it had rained, he understood the lack of people."

This sentence has an incorrect form of the mood. It should be "If it had rained, he would have understood the lack of people." This correction maintains consistency in the use of the conditional mood throughout the sentence.