【问】 That would be his mother.

这里为什么要用动词的虚拟语气?

【答】 这里的would并不是用来构成虚拟语气，而是表示“推测”、“猜想”，可以译为“可能”、“也许”、“想必”：

1. Who is the man you want to speak to?

You wouldn't know him.

你想要找谈话的那个人是谁?

你大概不认识(他)。

2. I thought you would have finished this by now. (Chambers)

我认为此刻你可能已经把这件事干完了。

3. He would have reached New York. (PEG)

想必他已经到达纽约了。

(参见will4)