Despite his efforts in practicing painting, the teacher still considered him untalented.

untalented 是由前缀 un-（表示“不、无”）和形容词 talented（有天赋的）组成的复合形容词。talented 来自名词 talent（天赋、才能）加上后缀 -ed 构成形容词。untalented 字面意思是“没有天赋的”，即“缺乏才能的”。