Who would have thought that this small startup would one day become a tech giant?

who 是疑问代词，表示"谁"，用于询问或引出疑问。 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"或"曾经"。 thought 是动词 think 的过去分词，意思是"认为、思考"。 who would have thought 是一个固定表达，字面意思是"谁会想到"，用来表示对出乎意料的事情感到惊讶。