Question

Then he cannot answer The question，I have to ask

someone easy，这里为什么用，Since

Answer 1

Since he cannot answer the question, I have to ask someone else. 

既然他回答不了这个问题，我只好去问别人。

Since 引导的是原因状语从句，通常是“显而易见”的原因。

比如：

Since he's old enough, you should let him take care of himself. 既然他已经长大了，你应该让他自己照顾自己。

Since this method doesn't work, let's try another. 

既然这个方法不行， 我们试试另一个。