per day是介词+名词,为\"介词短语\",作补充性状语,强调单位时间.
every day是形容词+名词,为\"名词短语\".名词一般不能作状语.
It is important that he should attend every day.
他每天都要出席，这很重要。
I go to the store most every day.
我几乎每天都去商店。
The output per day is over 10,000 tons.
日产过万吨。
水利万物而不争,
而万物莫能与之争。
