上面的句子倒是没有并列句,而是主从句。下面尝试从现在分词充当的成分做一些不成熟的解释。
从句为表示原因的状语从句时,如果将主句改为现在分词,这时候现在分词往往不是充当修饰性状语(逻辑主语为句子主语),而是充当补充性状语(逻辑主语为前面整句——因为前面整句表示的情况导致了后面的结果)。
所以上面的句子可以作如下转换(以句1为例):
Because he was defeated by his classmate, he felt discouraged.
-->
He was defeated by his classmate, making him feel discouraged. (虽然还是不如Defeated by ...句自然)
相当于
He was defeated by his classmate, which made him feel discouraged.