帮忙分析一下这个英语句子It is generally accepted that people benefit con

帮忙分析一下这个英语句子
It is generally accepted that people benefit considerably, both physically and mentally, from doing exercise; they need only to spend a certain time on regular exercise every week.
; 分号 请问怎么使用 在文中是什么意思? 还有 a certain time 为什么有 a, 作为时间来讲time是单数,但是不可数吧?作为次数讲,time可做复数,才可以用 a time, many times 对吧?为什么这里用 a