Noisy as it was, he went on reading as if nothing ______ .
为何答案是were happening而不是had happened?
这两个不都是虚拟语气吗？
是的，填 were happening 和 had happened 都是虚拟语气，但使用方法不同：用 were happening 表示当时正在发生的状况，用 had happened 表示针对当时来讲属已经发生的状况。对于你的句子来讲，非常显然 were happening 更符合句子语境，指“吵闹”的状况当时“正在发生”。句意：
Noisy as it was, he went on reading as if nothing were happening. 虽然非常吵，但他仍继续看书，仿佛啥事都没发生一样。（指当时本来非常“吵闹”，但对他来讲，仿佛当时“吵闹”的事没正在发生）