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When Spanish quantity words change their endings

In English, quantity words stay exactly the same no matter what they describe. You can have a lot of apples or be a lot happier, and the phrase a lot never changes. In Spanish, however, quantity words have a split personality.

Words like mucho (a lot), demasiado (too much), and poco (little) can act as two different parts of speech. Sometimes they describe nouns, and they must change their endings to match. Other times, they describe adjectives or verbs, and they freeze completely, never changing their form. Learning when to match the endings and when to leave them alone is a key step to sounding natural in Spanish.

Word list
demasiadoA1 too stays fixed with adjectives: Ella es demasiado alta (too tall), never demasiada mediomA1 half agrees with nouns, stays fixed with adjectives muchafA1 much agrees with feminine singular nouns muchasfA1 many agrees with feminine plural nouns muchomA1 much agrees with masculine singular nouns, stays fixed with verbs muchosmA1 many agrees with masculine plural nouns muyA1 very stays fixed with adjectives and adverbs pocasfA1 few agrees with feminine plural nouns pocomA1 little agrees with nouns, stays fixed as an adverb pocosmA1 few agrees with masculine plural nouns un pocoA1 a little stays fixed with adjectives bastanteA2 enough agrees with plural nouns cuántomA2 how much agrees with nouns you cannot count cuántosmA2 how many agrees with plural nouns you can count demasiadoA2 too much agrees with nouns, stays fixed with adjectives suficienteA2 enough agrees with plural nouns un poquitoA2 a little stays fixed with adjectives bastanteB1 quite stays fixed as an adverb
When quantity words describe nouns

When a quantity word tells you how much or how many of a noun (a person, place, or thing) there is, it acts like an adjective. This means it must agree in gender (masculine or feminine) and number (singular or plural) with the noun it describes.

For example, if you want to say too many questions, you look at the noun preguntas. Because it is feminine and plural, the word demasiado becomes demasiadas. The same rule applies to question words like cuánto (how much) and cuántos (how many).

Tenemos mucha agua para el viaje.
(We have a lot of water for the trip.)
Hay demasiadas preguntas en este examen.
(There are too many questions on this exam.)
¿Cuántos libros quieres leer?
(How many books do you want to read?)
When quantity words describe adjectives or verbs

When these same quantity words describe an adjective (like tired or expensive), another adverb, or a verb, they act as adverbs. Adverbs in Spanish never change their endings. They stay locked in their basic, singular masculine form (usually ending in -o).

Even if the people you are describing are a group of women, the quantity word stays exactly the same. You might be tempted to say demasiadas cansadas, but because tired is an adjective, the quantity word cannot change.

Las niñas están demasiado cansadas para jugar.
(The girls are too tired to play.)
Esta película es un poco aburrida.
(This movie is a bit boring.)
Mis padres trabajan poco los domingos.
(My parents work little on Sundays.)
The special case of muy and mucho

English speakers often confuse muy (very) and mucho (a lot). In Spanish, muy is strictly an adverb. It is used exclusively before adjectives and other adverbs, and it never changes form. On the other hand, mucho is used with nouns and verbs.

If you want to say you are very tired, you use muy because tired is an adjective. If you want to say you have a lot of work, you use mucho because work is a noun.

Estoy muy cansado hoy.
(I am very tired today.)
Tengo mucho trabajo en la oficina.
(I have a lot of work at the office.)
Words ending in -e

Words like bastante (enough, quite) and suficiente (sufficient) follow the exact same rules, but they look slightly different. Because they end in -e, they do not have separate masculine and feminine forms. However, they still add an -s when they describe a plural noun.

When used as adverbs before an adjective, they freeze and stay singular.

Tenemos suficientes platos para la fiesta.
(We have enough plates for the party.)
Estas preguntas son bastante difíciles.
(These questions are quite difficult.)
Summary

Here is the golden rule. If a quantity word describes a noun, treat it like an adjective and match its gender and number. If it describes an adjective, an adverb, or a verb, use the singular masculine form and leave it completely alone.