It happened that...使用方法

It happened that Yu Chunshun arrived in Nanjing on the evening ofDec.25th during his long journey on foot around the country.

请问老师it happened that在这里是什么使用方法？

it为形式主语，that为真的主语从句。happen在此为不及物动词，意为碰巧发生。翻译时译为：碰巧余纯顺某月某入做了某事。