It is suggested that sunscreen should be applied every one to two hours in a sunny day.建议在晴朗天气里，每隔一到两个小时涂抹防晒霜（本句中，that引导主语从句，it是形式主语）
It was yesterday that I came across one of my old classmates in senior high school.就是在昨天，我偶然遇到了一位我的高中同学（本句为强调句，强调时间状语yesterday，that是强调句的连接词）
This is the museum that I visited last year.这是我去年参观过的那家博物馆（本句，that引导定语从句，修饰先行词the museum，在从句中作visited的宾语）
