1. It was his first visit to his friend's house.

2. It is her first time to take part in the school club activity.

3. It is our duty to clean the room.

首先要分清楚：介词to 和不定式符号to。

句1，to his friend's house 是介词短语，作visit 的定语。这个不需要特别分析。to 表示“目的地；针对关系”。又如：a letter to the editor。

句2，to take part in the school club activity 是不定式短语，也是作定语，不是状语。它跟the first time 存在动状关系。说明“第一次参加学校社团活动”。

句3，It 是形式主语，to clean the room 是真正主语。跟前两句的明显区别是：句1，句2中的it 是人称代词，不是形式主语。可以用this 或that 代替it，意思不变。而句3不可以用this 或that 代替it。比较：

1. It/This/That was his first visit to his friend's house. （正）

2. It/This/That is her first time to take part in the school club activity.（正）

3. This/That is our duty to clean the room.（误）