At first I thought the task was difficult, but it became easier with practice.

at 是介词，表示"在...时候"或"处于...状态"。 first 是序数词，意思是"第一"或"最初的"。 at first 是一个固定短语，表示"起初"或"一开始"，用来描述事情最初的状态或看法。