“check in”，意思是“登记入住”，例如“I'll call the hotel.I'll tell them we'll check in tomorrow.”“我会给旅馆打电话，通知他们我们明天将会登记入住”。
还表示“办理登机手续”，例如“He had checked in at the airport for a flight to Atlanta.”“他在机场已经办理了飞往亚特兰大航班的登机手续”。
“check sth in（托运行李）”，例如“I said goodbye and went to check in my suitcases.”“我道别后就去托运行李了”。