两个简单句,合并为状语从句,they returned from a long walk成为了主句,结构保持不变.后面那句,把They were tired but happy改成Being tired but happy,作从句部分,但最终省略了being.所以就有了：,tired but happy.还有一个例句Jerry and I left Julie's room. We felt terribly relived.合并后为 Jerry and I left Julie's room terribly relieved. 两个短句合并后为什么没有加逗号都是伴随状语从句，只要放后面，加不加都可以。我觉得主要看写作习惯。或者简单点，靠语感。