fall in
phrasal verb with fall verb uk/fɔːl/ us/fɑːl/ fell | fallen
(ROOF)
（屋顶或天花板）坍塌，塌陷
If a roof or ceiling falls in, it drops to the ground because it is damaged.
Ten miners were trapped underground when the roof of the tunnel fell in. 坑道顶部坍塌，10名矿工被困地下。
(SOLDIERS)
列队;集合
If soldiers fall in, they form a line or move in a line, one behind the other.
"Company, fall in!" shouted the sergeant-major. “全连集合！”军士长喊道。
He started to march away, and the others fell in behind him. 他开始大步前进，其他人列队紧随其后。
Compare: fall out (SOLDIERS)