It didn't take him long to settle down at his new school.

it 在这里是形式主语，指代后面"to settle down at his new school"这件事。 settle 的意思是"安顿、安定"，指在一个新环境中找到舒适的状态。 down 在这里与 settle 搭配使用，settle down 是一个固定短语，表示"安定下来、适应新环境"。 settle down 合起来表示"适应新环境、安定下来"的意思。