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Quantity words: when to change their endings

In Portuguese, words that express quantity or degree can be tricky. Words like muito, pouco, and meio often play double duty. Sometimes they act as quantity words describing nouns. Other times, they act as adverbs describing verbs or adjectives.

Why does this matter? Because their job dictates whether they change their endings. If you learn when to change these words and when to freeze them, you will avoid some of the most common grammar mistakes in Portuguese.

Word list
demasiadoA1 too demasiado caras (unchanging adverb) vs demasiadas pessoas (agrees with noun) meiomA1 half meia hora (agrees with noun) vs meio cansada (unchanging adverb) muitaA1 much muita água (agrees with noun) vs muito bonita (unchanging adverb) muitasA1 many muita / muitas / muito / muitos (agrees with noun) muitoA1 very muito cansadas (unchanging adverb) muitomA1 much muito tempo (agrees with noun) vs muito cansada (unchanging adverb) muitosA1 many muitos livros (agrees with noun) vs muito cansados (unchanging adverb) poucasfA1 few poucas alunas / poucos alunos (agrees with noun) poucomA1 little pouca comida (agrees with noun) poucosmA1 few poucos alunos / poucas alunas (agrees with noun) bastanteA2 enough bastantes livros (agrees with noun) quantomA2 how much quanta água (agrees with noun) quantosmA2 how many quantas casas (agrees with noun) bastanteB1 quite bastante felizes (unchanging adverb)
Much, many, and very

When words like muito (much, many) and pouco (little, few) describe a noun, they are quantity words. This means they must agree in gender and number with the noun they describe.

Tenho muitas amigas na escola.
(I have many female friends at school.)
Há pouca água no copo.
(There is little water in the glass.)

However, when these exact same words describe an adjective or a verb, they act as adverbs to mean "very" or "not much". Adverbs do not change their form. They never change, always staying in their default masculine singular form.

As minhas amigas estão muito cansadas.
(My female friends are very tired.)
Nós dormimos pouco ontem.
(We slept little yesterday.)

Notice that we say muitas amigas because the word matches the noun, but muito cansadas because the word describes the adjective.

Half and kind of

The word meio follows the exact same logic, but it leads to frequent mistakes. When it means "half", it describes a noun and must agree with it.

Esperamos por meia hora.
(We waited for half an hour.)

But when meio means "kind of" or "somewhat", it is an adverb and never changes. Even if a woman is speaking about herself, she must use the default masculine form.

Ela está meio cansada.
(She is kind of tired.)

Saying meia cansada is a classic mistake. It would literally mean she is "half tired" as if she were a physical object sliced in two.

Enough and quite

The word bastante already ends in an "e", so it never changes for gender. But it does change for number when it acts as a quantity word describing a noun.

Compramos bastantes livros novos.
(We bought enough new books.)

If you use it to mean "quite" or "fairly" describing an adjective, it locks into its singular form.

Os livros são bastante caros.
(The books are quite expensive.)
How much and how many

Question words like quanto and quantos are also part of this family. When asking "how many" or "how much" of a noun, the word matches the noun.

Quantas pessoas vêm para a festa?
(How many people are coming to the party?)

When asking about a verb, it does not change.

Quanto custam as passagens?
(How much do the tickets cost?)
Summary

To know if you should change the ending of a quantity word, look at what it is attached to. If it describes a noun, it needs to match that noun in gender and number. If it describes an adjective or a verb, it is an adverb and stays in its default masculine singular form.