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'Máquina de lavar', 'sala de jantar' and other things named after their job

English often names an object after what it is for by putting an -ing word in front of it: a washing machine, a dining room, a sleeping bag. Portuguese does it the other way round. It starts with the object, adds de, and finishes with a plain infinitive: máquina de lavar (machine of to-wash), sala de jantar (room of to-dine), saco de dormir (bag of to-sleep).

English speakers tend to trip in two places. First, they reach for a Portuguese -ing form (lavando, dormindo), which sounds like something happening right now. Second, when the object becomes plural, they want to change the verb too. The infinitive never changes: one máquina de lavar, two máquinas de lavar.

Word list
sala de estarfA1 living room plural: salas de estar máquina de lavarfA2 washing machine plural: máquinas de lavar máquina de lavar roupafA2 washing machine plural: máquinas de lavar roupa sala de jantarfA2 dining room plural: salas de jantar aparelho de barbearmB1 razor plural: aparelhos de barbear ferro de engomarmB1 iron plural: ferros de engomar ferro de passarmB1 iron plural: ferros de passar máquina de barbearfB1 razor plural: máquinas de barbear máquina de lavar loiçafB1 dishwasher plural: máquinas de lavar loiça tábua de engomarfB1 ironing board plural: tábuas de engomar tábua de passarfB1 ironing board plural: tábuas de passar tábua de passar roupafB1 ironing board plural: tábuas de passar roupa camisa de dormirfB2 nightgown plural: camisas de dormir goma de mascarfB2 chewing gum plural: gomas de mascar máquina de escreverfB2 typewriter plural: máquinas de escrever tábua de cortarfB2 cutting board plural: tábuas de cortar fecho de corrermC1 zipper plural: fechos de correr lâmina de barbearfC1 razor blade plural: lâminas de barbear porta de correrfC1 sliding door plural: portas de correr saco de dormirmC1 sleeping bag plural: sacos de dormir canção de embalarfC2 lullaby plural: canções de embalar canção de ninarfC2 lullaby plural: canções de ninar rede de caçar borboletasfC2 butterfly net plural: redes de caçar borboletas rede de dormirfC2 hammock plural: redes de dormir
How it works

The pattern is always noun + de + infinitive. The noun at the front is the real object, and it controls everything: gender, plural and any adjectives. The infinitive after de is a frozen label that only says what the object is for. So you write as máquinas de lavar and as salas de jantar, adding the plural ending only to the first word.

As máquinas de lavar estão na outra sala.
(The washing machines are in the other room.)

Adjectives also agree with the front noun and usually come after the whole expression: uma máquina de lavar nova (a new washing machine). The rule of thumb: if English has a ___ing thing meaning a thing for doing something, try thing + de + infinitive. The same logic gives you óculos de ler (reading glasses).

The words
Rooms

Sala de estar (living room, literally room for being) and sala de jantar (dining room) are the ones you meet first.

Compramos uma mesa para a sala de jantar.
(We bought a table for the dining room.)
Vemos televisão na sala de estar.
(We watch TV in the living room.)
Around the house

Máquina de lavar and the fuller máquina de lavar roupa both mean washing machine. A dishwasher is máquina de lavar loiça in Portugal. For the iron, Portugal says ferro de engomar and Brazil says ferro de passar. Add tábua de cortar (cutting board) and the old-fashioned máquina de escrever (typewriter).

A máquina de lavar roupa faz muito barulho.
(The washing machine makes a lot of noise.)
Preciso de um ferro de engomar novo.
(I need a new iron.)
Corta a cebola na tábua de cortar.
(Chop the onion on the cutting board.)
A minha avó ainda tem uma máquina de escrever.
(My grandmother still has a typewriter.)
Shaving

A razor is aparelho de barbear or máquina de barbear. The blade inside is a lâmina de barbear.

Esqueci-me do aparelho de barbear em casa.
(I left my razor at home.)
Sleep

Dormir is a favorite: camisa de dormir (nightgown), saco de dormir (sleeping bag) and rede de dormir (hammock). A lullaby is canção de embalar in Portugal and canção de ninar in Brazil.

Vamos levar os sacos de dormir para o acampamento.
(We're taking the sleeping bags to the campsite.)
As crianças adormeceram nas redes de dormir.
(The children fell asleep in the hammocks.)
A mãe canta uma canção de embalar ao bebé.
(The mother sings a lullaby to the baby.)
Odds and ends

Goma de mascar is chewing gum, and a collector's butterfly net is a rede de caçar borboletas.

Não deites a goma de mascar no chão.
(Don't throw the chewing gum on the ground.)
Watch out
Purpose or manner

Most of these name what the object is for. A few name how it works instead. A porta de correr is not a door for running. It is a door that slides. A fecho de correr (zipper, in Portugal) works the same way: it is a fastener that runs along a track.

Fecha a porta de correr, por favor.
(Close the sliding door, please.)
O fecho de correr do meu casaco está estragado.
(The zipper on my coat is broken.)
The infinitive can carry an object

In máquina de lavar loiça and rede de caçar borboletas, the thing being washed or caught comes after the verb, just as in a normal sentence. It stays singular or plural as meaning requires, whatever happens to the front noun.

Gender comes from the front noun

O saco de dormir is masculine and a rede de dormir is feminine, although both end in dormir.

The typical mistake is changing the verb in the plural or using an -ing form. Say salas de jantar, never sala jantando, and never let the verb take a plural ending.

Summary

To name an object by its job, say the object first, then de, then a plain infinitive: máquina de lavar, sala de jantar, saco de dormir. Only the front noun changes for plural, gender and adjectives (as máquinas de lavar novas). The infinitive stays exactly as it is.