1. an average of＋复数名词作主语，谓语用复数；而the average of＋复数名词作主语，谓语通常用单数。如：

误：An average of 10 students was absent each day.

正：An average of 10 students were absent each day.

2. on (the) average是固定短语，不用其他介词。如：

On (the) average we receive ten letters each day.

3. average 还表示“一般的、普通的、寻常的”，例如：

I was only average academically.