“胜败乃兵家常事”用英语可以这样表达：you can't win them all，它表示：

1、indicates the reality that it is not possible to always succeed
表明不可能总是成功

2、failure is inevitable and is bound to happen
失败是不可避免的，也是必然发生的

解析：be bound to do/be sth 表示一定会，相当于 certain.

3、implies that it is impossible to always keep succeeding without disappointment
意味着不可能总是成功而不失望

例如：

1、To be very frank, I am very disappointed that they did not give me the post of the manager, as I feel I truly deserved it after 5 years of work experience. Oh well, you can't win them all you see.
坦率地说，我很失望他们没有给我经理的职位，因为我觉得凭我5年的工作经验，我真的是当之无愧。哦，好吧，你不可能拥有所有你看到的。

解析：give sb sth 是双宾语结构，相当于 give sth to sb.

2、Obviously, I would have loved to win the national trophy for the school this year as well, but you just can't win them all.
很明显，我也很想为学校赢得今年的国家杯，但年年赢得国家杯是不可能的。

解析：as well 是一个副词短语，意为“也；还”，相当于 also，too 和 in addition，经常放在句尾。

3、I expected to come first this semester as I had been scoring really well in all the exams but I guess you can't win them all.
我本以为这学期会得第一名，因为我所有的考试都考得很好，但我想每次都得第一名是不可能的。