Who would have thought that buying a house could take so long!

who 是疑问代词，表示"谁"，用于询问或引出疑问句。 would 是情态动词 will 的过去式，在这里表示假设或推测的语气。 have 是助动词，与过去分词 thought 构成完成时态，表示"已经"。 thought 是动词 think 的过去分词，意思是"想、认为"。 who would have thought 是一个固定表达，字面意思是"谁会想到"，用来表示对某件事感到惊讶或出乎意料。