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When things change by themselves, add 'se'

In English, many verbs work in two directions with no change at all: you close the door, or the door simply closes. Portuguese usually marks the second use, where the subject goes through the change without anyone doing it, with the pronoun se. You say fechei a porta (I closed the door), but a porta fechou-se (the door closed). The same pattern shows up with apagar-se, espalhar-se, derreter-se, afundar-se and many more.

English speakers trip over this in two ways. They leave out se because English has nothing there, or they add it to the version with an object, where it does not belong. This article lists the verbs that work like this and shows both sides of each pair.

Word list
fecharA1 to close fechar a porta = to close the door mexer-seA2 to move não te mexas = don't move mover-seA2 to move não se mova = don't move adaptar-seB1 to adapt ele adaptou-se à nova cidade = he adapted to the new city apagar-seB1 to go out a luz apagou-se = the light went out aquecerB1 to warm up aquecer a sopa = to heat up the soup aquecer-seB1 to warm up a sala aquece-se = the room warms up espalharB1 to spread espalhar os papéis = to scatter the papers esquentarB1 to warm up esquentar a comida = to warm up the food esquentar-seB1 to warm up a água esquentou-se = the water warmed up estragarB1 to ruin estragar a surpresa = to ruin the surprise fechar-seB1 to close a porta fechou-se = the door closed moverB1 to move mover a mesa = to move the table adaptarB2 to adapt adaptar a casa às necessidades = to adapt the house to one's needs afundarB2 to sink afundar o navio = to sink the ship afundar-seB2 to sink o navio afundou-se = the ship sank apagarB2 to put out apagar o fogo = to put out the fire avariar-seB2 to break down o carro avariou-se = the car broke down concretizarB2 to realize concretizar o projeto = to make the project a reality concretizar-seB2 to come true o plano concretizou-se = the plan came true curarB2 to cure curar o doente = to cure the patient curar-seB2 to heal a ferida curou-se = the wound healed derreter-seB2 to melt o gelo derreteu-se = the ice melted descongelar-seB2 to melt a comida descongelou-se = the food thawed espalhar-seB2 to spread a doença espalhou-se = the disease spread estragar-seB2 to spoil o leite estragou-se = the milk went off estragar-seB2 to break down a máquina estragou-se = the machine broke down extinguirB2 to put out extinguir o incêndio = to extinguish the fire extinguir-seB2 to go out o fogo extinguiu-se = the fire went out incendiar-seB2 to catch fire a casa incendiou-se = the house caught fire realizarB2 to realize realizar um sonho = to make a dream come true realizar-seB2 to come true o sonho realizou-se = the dream came true transformar-seB2 to become a lagarta transformou-se em borboleta = the caterpillar turned into a butterfly agravarC1 to worsen agravar o problema = to make the problem worse agravar-seC1 to worsen a situação agravou-se = the situation worsened difundir-seC1 to spread o pânico difundiu-se = the panic spread incendiarC1 to set fire incendiar o carro = to set the car on fire propagar-seC1 to spread a notícia propagou-se = the news spread unirC1 to unite unir as peças = to join the pieces unir-seC1 to unite os países uniram-se = the countries united
How it works

Each verb has two constructions:

Someone causes the change

Subject + verb + object, with no se: fechar a janela, apagar o fogo, afundar o navio.

The thing changes on its own

The thing becomes the subject and the verb takes se: a janela fechou-se, o fogo apagou-se, o navio afundou-se.

Rule of thumb: if you could add by itself to the English sentence, reach for se. In European Portuguese the pronoun normally follows the verb (fechou-se). In Brazil it goes before it (se fechou), and in everyday speech Brazilians often drop it completely: a porta fechou, a luz apagou. Both are correct in their own variety. What never works is se together with a direct object.

O João fechou a janela.
(João closed the window.)
A janela fechou-se com o vento.
(The window closed in the wind.)
The words
Light and fire

You apagar or extinguir a fire or a light, and it apaga-se or extingue-se by itself. Extinguir is more formal. Something that catches fire incendeia-se.

Os bombeiros extinguiram o fogo, mas a casa vizinha já se tinha incendiado.
(The firefighters put out the fire, but the house next door had already caught fire.)
Durante a tempestade, as luzes apagaram-se.
(During the storm, the lights went out.)
Closing and moving

Besides fechar and fechar-se, there is mover (to move something) and mover-se or mexer-se (to move, change position).

Ajudas-me a mover a mesa? Ela não se move sozinha.
(Will you help me move the table? It won't move by itself.)
Temperature

You aquecer the soup (Brazil: esquentar), and a room aquece-se or esquenta-se. Ice derrete-se and frozen food descongela-se.

O gelo derreteu-se ao sol.
(The ice melted in the sun.)
A sala aquece-se depressa com a lareira.
(The living room warms up quickly with the fireplace.)
Spreading and uniting

You espalhar papers on a table, but news or a disease espalha-se. Propagar-se and difundir-se mean the same in a more formal style. You unir pieces, and people or countries unem-se.

A notícia espalhou-se pela aldeia num só dia.
(The news spread through the village in a single day.)
Os dois partidos uniram-se contra a proposta.
(The two parties united against the proposal.)
Things going wrong or getting better

You estragar something (ruin it), and food or a machine estraga-se. A car avaria-se (mainly European Portuguese). You afundar a ship and it afunda-se. You agravar a problem and it agrava-se. A doctor can curar you, and a wound cura-se. A dream you fulfil realiza-se.

O leite estragou-se porque ficou fora do frigorífico.
(The milk went off because it was left out of the fridge.)
O carro avariou-se na autoestrada.
(The car broke down on the motorway.)
O navio afundou-se perto da costa.
(The ship sank near the coast.)
A situação agravou-se durante a noite.
(The situation got worse during the night.)
A ferida curou-se em poucos dias.
(The wound healed in a few days.)
Watch out
Not every change-of-state verb takes se

Melhorar, aumentar and ferver change on their own without it: a situação melhorou, a água ferveu. Compare a situação agravou-se. Learn these one by one.

Some verbs can go either way

With weather, aquecer often appears without se: o tempo aqueceu. Classic cases outside this list, like abrir-se and quebrar-se, behave the same as fechar-se.

Brazilian speech drops se

You will hear o navio afundou and a comida estragou. Understand them, but in European Portuguese keep the pronoun.

People versus things

With a person as subject, mover-se and mexer-se feel more like true reflexives: não se mexa! means don't move!

The typical mistake

Never combine se with an object: say apaguei a luz, not apaguei-me a luz.

Summary

When someone makes the change happen, use the plain verb with an object: fechar a porta, apagar a luz. When the thing changes by itself, make it the subject and add se: a porta fechou-se, a luz apagou-se. If English could add by itself, Portuguese usually wants se, although Brazilians often leave it out in speech.