核心问题
That would be his mother.
这里为什么要用动词的虚拟语气？
词典解答
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”:
-
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)。
-
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。
-
He would have reached New York. (PEG) 想必他已经到达纽约了。
(参见 will )