1.The shy boy appeared in the classroom,_____ his mother,who was leading him _____hand.
A.你可以把句子补全为：
The shy boy appeared in the classroom and he followed his mother,who was leading him by the hand.
这时你会看到and he followed his mother中的he 就是the shy boy,为了避免重复,把后面的and he followed 变为非谓语following .
B.by the hand 是固定用法.
又如：The man hit Bob on the head.
Don't pull me by the collar.