他在书桌旁坐好后，他母亲开始给他讲故事。（seating himself at the desk拥有了自己的逻辑主语he，注意是“主格”）
= When he seated himself at the desk, his mother began to tell him a story.
The key to the bike lost, he had to walk to school.
由于丢了自行车钥匙，他只好步行去学校。（lost的逻辑主语是the key，lost也可以用完成式having been lost）
= Because the key to the bike had been lost, he had to walk to school.
