ADV
首先;第一
You use firstly in speech or writing when you want to give a reason, make a point, or mention an item that will be followed by others connected with it.
例句
The programme is now seven years behind schedule as a result, firstly of increased costs, then of technical problems...
该项目现已经比预定时间晚了7年，原因首先是因为成本增加，其次是由于技术问题。
Vitamin C has many roles to play in weight control. Firstly, it is needed for hormone production.
维生素C在控制体重方面扮演着多重角色。首先，荷尔蒙的生成有赖于它。