USAGE NOTE 用法说明: first, first of all and 和 at first
Use first and first of all at the beginning of a sentence to talk about the first or most important thing in a series of things. first 和 first of all 用在句首，谈论一系列事（物）中的第一个或最重要的一个:
First, chop the onions. 首先，将洋葱切碎。
First of all, I'd like to thank everyone for coming today. 首先，我要感谢今天大家到场。
Use at first to talk about what happened at the beginning of an event or situation, especially when this changes later. at first 用于谈论事件或形势一开始的情形，尤其是稍后有变化的时候:
We really liked the hotel at first, but then they started the building work. 起初，我们真的很喜欢那家酒店，但接着他们便开始了改建。