原文语法结构分析
一、不定式作主语
To eat out was also a social activity.
不定式 “To eat out” 是句子的 主语，谓语是 was。
注意：虽然英语中通常避免使用不定式直接作主语导致头重脚轻，因此常用 “It” 作形式主语，如：It is a social activity to eat out.
二、不定式作表语
My plan is to invite my friends over at the weekend…
不定式 “to invite my friends…” 作 表语，说明主语 “My plan” 的内容，跟在系动词 is 后。
It was not that difficult to make a simple and tasty dish.
这是一个 形式主语结构，其中真正的主语是 不定式 “to make a simple and tasty dish”，it 为形式主语。结构：It + be + adj. + 不定式。
三、原文其他不定式用法
不定式作宾语
I was eager to try new restaurants.
不定式作宾语补足语
…allow me to enhance the relationship…
不定式作后置定语
…a variety of recipes to choose from.
不定式作目的状语
…to show them my new-found skills.
不定式在形式主语结构中作真正主语
It was not that difficult to make a simple and tasty dish.