He adapted to the new job so quickly; it felt like he was part of the team before you know it.

before 是介词或连词，表示"在...之前"的时间关系。 you 是第二人称代词，指"你"或"你们"。 know 是动词，意思是"知道、了解"。 before you know it 是一个固定短语，字面意思是"在你意识到之前"，引申为"转眼间、很快地"表示事情发生得特别迅速。