Only after what seemed a long time did rescue come to him .

如果after为介词,what用作宾语.但如果把after理解为连词,后面应该接句子,用it作主语,指时间不也说得通么,原句可还原为Rescue came to him only after it seemed a long time .谁能告诉我it为什么不行

如果有it，it seemed a long time,就变成了另一个句子了，但是，这里只是说时间上过了好像很长时间。

Rescue did come to him only after a long time.

what seemed 是形容a long time的

关键在与what在这里是“双重身份”，即既做“after”的宾语，又做“seemed”的形式主语（如果不是双重成分则用it）。做双重成分的永远都只有一个what,记住了，屡试不爽。