Final answer:
The sentence "If it had rained, he understood the lack of people." has an incorrect form of the mood.
Step-by-step explanation:
The sentence "If it had rained, he understood the lack of people." has an incorrect form of the mood. The correct form of the sentence should be "If it had rained, he would have understood the lack of people." The incorrect sentence does not properly use the conditional mood to convey a hypothetical situation in the past.
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