to line up, to get into formation, to form, to develop
El lunes por la tarde empezaron a formarse filas frente a la Torre.
On Monday afternoon, lines started forming in front of the Tower.
Esos presos a formarse.
Those prisoners, line up.
— Nosotros los pobres (1948)
Esas visitas a formarse, a pasar su lista.
Those visits to get in line, to check in.
Suele formarse niebla cuando hay humedad y baja la temperatura.
Fog usually forms when there is humidity and the temperature drops.