Didn't you see the sign on the door? I put it up yesterday.

Didn't 是助动词 do 的过去式否定形式，置于句首构成否定疑问句。这种句式通常带有预设——说话人认为对方理应看到却未看到，传达出惊讶或轻微不满的语气。在助动词 didn't 之后，主动词 see 使用原形，时态由 didn't 承载。