lápis/ônibus/vírus

Invariable Plural Nouns

Level 2

Some Portuguese nouns ending in -s are identical in both singular and plural forms. The number is shown only by the words around them (like o/os or um/dois).

How It Works

In Portuguese, most nouns form the plural by adding -s or -es. However, a specific group of nouns is invariable, meaning the word itself never changes.

1. The Stress Rule
This pattern applies to nouns ending in -s or -x that are:

  • Paroxítonas: Stressed on the second-to-last syllable (e.g., lápis, tênis, vírus, bônus, tórax, látex).
  • Proparoxítonas: Stressed on the third-to-last syllable (e.g., ônibus).

2. How to tell if it's plural?
Since the noun doesn't change, you must look at the definite articles, indefinite articles, possessive adjectives, demonstrative adjectives, or numerals (dois lápis) to know the quantity.

3. Stressed Endings (The Contrast)
If the word ends in a stressed -s (oxítonas or monosyllables), it is usually not invariable. These words follow the -es plural rule. For example: o mês becomes os meses. Note that words like mês, português, or freguês lose their graphic accent in the plural (meses, portugueses, fregueses).

4. Exceptions
A few monosyllables ending in -s are invariable, such as o cais (the pier/piers) and o xis (the letter X/Xs).

Examples (6)

  1. O meu lápis está quebrado.

    My pencil is broken.

    Word by Word: The my pencil is broken.

    Here, 'lápis' is singular, indicated by the singular article 'o' and possessive 'meu'.

  2. Eu comprei dois lápis novos.

    I bought two new pencils.

    Word by Word: I bought two pencil new.

    Even though we are talking about two pencils, the word 'lápis' remains exactly the same.

  3. O ônibus chega às oito horas.

    The bus arrives at eight o'clock.

    Word by Word: The bus arrives at the eight hours.

    'Ônibus' is a proparoxítona (stressed on 'ô'), so it is invariable.

  4. Muitos ônibus circulam nesta avenida.

    Many buses circulate on this avenue.

    Word by Word: Many buses circulate in this avenue.

    The word 'muitos' (many) tells us that 'ônibus' is plural here.

  5. Este vírus é muito perigoso.

    This virus is very dangerous.

    Word by Word: This virus is very dangerous.

    'Vírus' is another common invariable noun ending in -s.

  6. Os médicos estudam vários vírus.

    The doctors study several viruses.

    Word by Word: The doctors study several virus.

    The plural is marked by 'os' and 'vários', not by the noun itself.

Common Mistakes

  • Adding '-es' to unstressed endings (over-regularization).

    Eu tenho dois lápises.
    Eu tenho dois lápis.

    Learners often try to apply the rule for words like 'mês/meses' to 'lápis', but 'lápis' is unstressed at the end and does not change.

  • Failing to pluralize the article or adjective.

    O ônibus estão atrasados.
    Os ônibus estão atrasados.

    Because the noun doesn't change, you must be extra careful to pluralize the article ('os') and the verb ('estão') to show plurality.

How It Connects

Tap a pattern to see what it builds on and what it leads to.

lápis/ônibus/vírusLevel 2Invariable Plural Nouns

Learn This First

To understand invariable plural nouns, you first need to know the standard pluralization rules in Portuguese. Since these nouns do not change their spelling between singular and plural, you must rely on articles and adjective agreement to identify the number of the noun in a sentence.