89.A three-year-old might need help in understanding that other children feel pain just as he does, and that hitting a playmate over the head with a heavy toy requires an apology.
主干：A three-year-old might need help
解析：in understanding... 是介词短语作状语，表示在明白...方面。understanding后面有两个that 引导的宾语从句。前一个从句中，as he does 引导的是比较状语从句，修饰feel pain。后一个从句中，hitting...是动名词作主语，谓语动词requires用单数形式。
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