几个难区别的英语单词."average" "ordinary" "normal" "common"这几个词到底怎么区别啊

average
n.平均数,平均水平
adj.一般的,通常的,平均的
v.取平均值,达到平均水平
The average of 4,6 and 8 is 6.
4,6,8的平均数是6.
ordinary
adj.普通的
Now electrical appliances have entered into ordinary families.
现在家用电器已经步入普通家庭.
normal
adj.正常的,正规的
n.常态,标准
I really don't know what kind of man he really is; on one occasion,he is normal,and on another,he acts like a lunatic.
我真不了解他到底是怎样的人; 一会儿正常,一会儿又像个疯子.
common
adj.共同的,平常的,普通的
n.平民,普通,公地,公园
He's not an officer,but a common soldier.
他不是军官而是普通士兵.