故事的上下文是这样的：

“Fancy seeing you here, Professor McGonagall.” He turned to smile at the tabby, but it had gone. Instead he was smiling at a rather severe-looking woman who was wearing square glasses exactly the shape of the markings the cat had had around its eyes. She, too, was wearing a cloak, an emerald one. Her black hair was drawn into a tight bun. She looked distinctly ruffled. “How did you know it was me?” she asked. “My dear Professor, I’ve never seen a cat sit so stiffly.” “You’d be stiff if you’d been sitting on a brick wall all day,” said Professor McGonagall. “All day? When you could have been celebrating? I must have passed a dozen feasts and parties on my way here.” Professor McGonagall sniffed angrily. “Oh yes, everyone’s celebrating, all right,” she said impatiently. “You’d think they’d be a bit more careful, but no — even the Muggles have noticed something’s going on. It was on their news.” She jerked her head back at the Dursleys’ dark living-room window. “I heard it. Flocks of owls . . . shooting stars. . . . Well, they’re not completely stupid. They were bound to notice something. Shooting stars down in Kent — I’ll bet that was Dedalus Diggle. He never had much sense.” “You can’t blame them,” said Dumbledore gently. “We’ve had precious little to celebrate for eleven years.” “I know that,” said Professor McGonagall irritably. “But that’s no reason to lose our heads. People are being downright careless, out on the streets in broad daylight, not even dressed in Muggle clothes, swapping rumors.” She threw a sharp, sideways glance at Dumbledore here, as though hoping he was going to tell her something, but he didn’t, so she went on. “A fine thing it would be if, on the very day You- Know-Who seems to have disappeared at last, the Muggles found out about us all. I suppose he really has gone, Dumbledore?” 

从上文可知，这句话是Prof. McGonagall说给Dumbledore的。句中的You-Know-Who指神秘人。这个句子是个虚拟条件句，表示与将来事实相反的假设。主句使用了完全倒装句，还原正常语序则为It would be a fine thing if...

a fine thing是反语，字面上意思为“美妙的事”，实为说反话，表达的意思”要是那样，他们就高兴了（他们就知道他们错了）”。

由于条件从句是对将来情况的假设，主句是根据这个假设的推理，故从句为if the Muggles found out about us all，主句为it would be a fine thing. 主句和从句的谓语都是与将来事实相反的虚拟语气形式。其中if从句带有时间状语从句，该从句由 on the very day 引导，由于这个时间状语从句表示的是将来可能的事实，故从句用陈述语气，即用一般现在时表示将来: seems to have dissappeared.

句子参考译文：

“如果在神秘人似乎消失的那天麻瓜们发现了我们所有人的情况，那可太妙了！我想他实际上已经消失了。你在听我说吗？“