In English, adding an 's' to the end of a word is the most common way to make it plural. Because of this, English speakers learning Portuguese often see a final 's' and automatically assume the word is plural. However, Portuguese has several everyday nouns that end in 's' even in their singular form.
For example, lápis means pencil, and ônibus means bus. These words are not plural, they simply have an 's' built into their base spelling. To know if you are talking about one pencil or multiple pencils, you have to look at the words around the noun, such as the articles or adjectives.
Many common singular nouns ending in 's' have only one form. This means their spelling does not change at all when they become plural. You use the exact same word for one item or fifty items. To show the difference, you change the article from singular to plural, like moving from o or um to os or dois.
Words in this category include lápis, ônibus, vírus, pires, bónus, and atlas.
Eu perdi o meu lápis.
(I lost my pencil.)
Aqueles lápis são muito velhos.
(Those pencils are very old.)
Eles entraram no ônibus errado.
(They got on the wrong bus.)
O laboratório estuda um vírus perigoso.
(The lab is studying a dangerous virus.)
In Portugal, you will often eat a sandes. Just like the others, it remains the same whether you order one or three.
Eu comi uma sandes de queijo.
(I ate a cheese sandwich.)
Compound nouns
Portuguese has a very common pattern for creating new nouns. It takes a verb and attaches a plural noun to it. Because the second half of the compound is plural, the resulting word always ends in 's', but it usually refers to a single object or person.
For instance, a saca-rolhas is a tool that pulls out corks, and a guarda-costas is a person who guards backs. These compound words also have only one form. You can identify if they are singular or plural by looking at the article.
Ele usou o saca-rolhas para abrir o vinho.
(He used the corkscrew to open the wine.)
O guarda-costas ficou perto da porta.
(The bodyguard stayed near the door.)
O meu carro tem um para-brisas partido.
(My car has a broken windshield.)
Other common examples of this pattern include porta-malas, lava-louças, abre-garrafas, micro-ondas, and paraquedas.
Uncountable nouns and the typical trap
Some nouns ending in 's' are uncountable nouns, meaning they cannot easily be counted and are almost always treated as singular. This includes diseases like diabetes and sports like tênis.
A diabetes é uma doença crónica.
(Diabetes is a chronic disease.)
Ela joga tênis aos fins de semana.
(She plays tennis on weekends.)
Finally, a warning. Do not assume that every singular noun ending in 's' is invariant. This rule about words not changing usually applies to words where the final syllable is unstressed, like ônibus or lápis. If the stress falls heavily on the final 's' syllable, the word usually has a normal plural form. For example, mês (month) becomes meses, and país (country) becomes países.
Summary
When a Portuguese word ends in an unstressed 's', or is a compound word ending in a plural noun, it might be a singular noun. Because these words have only one form, their spelling does not change in the plural. Always look for clues like o versus os to figure out how many items are being discussed.
Singular nouns ending in 's'