if you become rich,you will have a difficult time knowing who your real friends are.请问know为什么加ing然后那个difficult time，不是困难时间吗，在这咋翻译，然后再说出整句翻译,

have a hard / difficult time + (in) + 动词ing.
----- she has a hard time when she does such work
=== she has a hard time (in) doing such work
一目了然，简单扼要

远不只是 time 这个词的。 以下句子均正确。可自己观察总结一下：
I had an easy morning taking the test.
Did you have a good time travelling around South America?
You'll have a terrible week working on that project.
I had a bit of trouble finding his house.
She had no difficulty finishing her homework.
I had a great evening talking with you.

have +a +adj.+time 这个句型后面是加-ing.形式,译文是：如果你变得富有了,你将很难知道谁是你真正的朋友

you will have a difficult time knowing这一部分中有两个动词。
则knowing为非谓语动词.know在句中表主动.所以加ing
如果你变得富有，你将很难知道谁是你真正的朋友.
difficult time不用刻意的去翻译.have difficult time 表困难.