有个同学问到这个问题，在列举一系列行为时，究竟是该用 first, second, third .... 还是用 firstly, secondly, thirdly .... ?
其实，在美国英语里，这两种都可以，意思没差别。不过 firstly, secondly, thirdly ... finally 太正式一点; 而first， second, third ... 更加口语化一点，也更加常见。
例如：
First, pour one liter of water into the pot.
Second, put 1kg of rice into the pot.
Third, shake the pot to make sure the water cover all the rice.
以上流程也可以写成：
Firstly, pour one liter of water into the pot.
Secondly, put 1kg of rice into the pot.
Thirdly, shake the pot to make sure the water cover all the rice.
所以其实两种系统都可以用，但是美国人一般不混用。
不过在英国英语里，人们会说：first（或者 first of all）, secondly, thirdly .... last（注意不是 lastly）.
我们中国人学用英语，可以怎么简单怎么来，建议同学们或者用 first, second, third, .... 或者用 firstly, secondly, thirdly. 不要混用。
另外，表示顺序的副词还有 additionally, eventually, subsequently, finally 等等。