His friends always pay him a compliment on his piano playing.

pay 在这里是动词，意思是"给予、付出"。 a 是不定冠词，用于单数可数名词前，表示"一个"。 compliment 是名词，意思是"称赞、恭维"。 pay a compliment 是一个固定短语，意思是"给予赞美、夸奖"。