This is the reason why we must cherish the time.把time前面的the去掉,不需要特指.
Nobody knows what time is like,for we cannot see it,nor can we contact it.for前面不需要加逗号,for表示因为的时候,不需要逗号的.（PS：感觉你这句话有点怪怪.）
Time is abstract,which we can only envisage in our brains.个人认为把brains改成minds会好一点.既然是抽象,那么minds会适合.
When a people dies,his time has ended.(前后语态一致好点,when someone dies,his time ends.)