How do you form the superlative in Spanish?

Use the definite article (el, la, los, las) plus mas (most) or menos (least) plus the adjective: el restaurante mas caro (the most expensive restaurant), la opcion menos buena (the least good option). To say in after a superlative, Spanish uses de, not en: el mejor cafe de la ciudad (the best coffee in the city). The article agrees in gender and number with the noun, and so does the adjective.

What are the irregular superlatives in Spanish?

Four adjectives skip the mas/menos pattern and use a special word. Bueno becomes el mejor (the best), malo becomes el peor (the worst), grande becomes el mayor (the biggest or oldest), and pequeno becomes el menor (the smallest or youngest). You never say el mas bueno or el mas malo for these - the irregular form does the whole job: el mejor dia de mi vida (the best day of my life).

What does the -isimo ending mean?

The -isimo ending is the absolute superlative. It means very or extremely, without comparing the thing to anything else. Bueno becomes buenisimo (extremely good), caro becomes carisimo (extremely expensive), facil becomes facilisimo (extremely easy). You drop the final vowel and add -isimo, and it agrees for gender and number like any adjective: una casa carisima, unos libros buenisimos.

Spanish Superlatives

A superlative names the extreme of a group: the best, the worst, the most, the least. Spanish has a clean pattern for it, four irregular forms to learn, and a separate ending - -isimo - for saying something is extreme without comparing it to anything.

The basic pattern: article + mas/menos + adjective

The frame is simple:

el / la / los / las + mas (most) or menos (least) + adjective

el restaurante mas caro (the most expensive restaurant)
la calle menos tranquila (the least quiet street)
los cafes mas baratos (the cheapest cafes)
las casas menos grandes (the least big houses)

The article and the adjective both agree with the noun in gender and number. If the noun is already on the table, you can drop it and the article carries the load: De todos, este es el mas barato (Of all of them, this is the cheapest).

The de trap: "in" becomes de

English says "the best in the city". Spanish does not use en here. It uses de:

el mejor cafe de la ciudad (the best coffee in the city)
la chica mas alta de la clase (the tallest girl in the class)
el dia mas frio del ano (the coldest day of/in the year)

This catches almost everyone, because the English instinct is to translate "in" as en. After a superlative, it is de, every time.

The four irregular superlatives

Four common adjectives ignore the mas/menos frame and use a dedicated word instead. The same word is both comparative ("better") and, with the article, superlative ("the best").

Adjective	Comparative	Superlative	Meaning
bueno	mejor	el / la mejor	the best
malo	peor	el / la peor	the worst
grande	mayor	el / la mayor	the biggest / oldest
pequeno	menor	el / la menor	the smallest / youngest

Es el mejor cafe de la calle. (It's the best coffee on the street.)
Fue el peor dia de mi vida. (It was the worst day of my life.)
Mi hermano mayor vive en Madrid. (My older brother lives in Madrid.)
La menor de las tres es muy lista. (The youngest of the three is very clever.)

Note the split with grande and pequeno: mayor and menor usually mean older/younger or greater/lesser, while plain el mas grande and el mas pequeno are used for physical size. Mi hermano mayor is my older brother; el edificio mas grande is the biggest building.

You do not say el mas bueno or el mas malo for quality. The irregular form does it: el mejor, el peor.

The absolute superlative: -isimo

There is a second kind of superlative that compares the thing to nothing at all. It just means very or extremely, dialled up. You build it by dropping the final vowel of the adjective and adding -isimo (with the matching gender and number ending).

bueno to buenisimo (extremely good)
caro to carisimo (extremely expensive)
grande to grandisimo (huge)
facil to facilisimo (dead easy)

It agrees like any adjective: una casa carisima, unos cafes buenisimos, unas preguntas facilisimas.

El cafe es buenisimo and el cafe es muy bueno mean roughly the same thing - buenisimo just hits harder.

The spelling changes

This is where -isimo bites. Some endings have to change to keep the sound right:

-co to -quisimo: rico to riquisimo (the c would soften before i, so it becomes qu).
-go to -guisimo: largo to larguisimo (the g would soften, so it becomes gu).
-z to -cisimo: feliz to felicisimo (z always becomes c before i in Spanish).
words ending in -ble to -bilisimo: amable to amabilisimo.

So "the cheapest" as an absolute - really cheap - is baratisimo, but "really tasty" is riquisimo, not the wrong ricisimo.

Worked examples

Este es el cafe mas barato de la zona. (This is the cheapest cafe in the area.) - regular pattern, de for "in".
Aquel restaurante es el peor de la ciudad. (That restaurant over there is the worst in the city.) - irregular peor.
La tarta esta buenisima. (The cake is absolutely delicious.) - absolute -isimo.
Es la chica menos seria de la clase. (She's the least serious girl in the class.) - menos plus de.
Mi abuela es mayor que mi madre. (My grandmother is older than my mother.) - mayor as a plain comparative.

Common mistakes English speakers make

Using en instead of de. "The best in the world" is el mejor del mundo, never el mejor en el mundo. After a superlative, "in" is de.

Saying el mas bueno. For "the best", it is el mejor. For "the worst", el peor. The regular mas pattern does not apply to bueno and malo when you mean quality.

Forgetting the -isimo spelling change. "Really rich/tasty" is riquisimo, not the wrong ricisimo. "Really happy" is felicisimo, not felizisimo. Watch the c, the g and the z.

Dropping the agreement. The article, the adjective and the -isimo form all agree with the noun: las casas mas caras, unas tartas buenisimas. A single masculine ending stuck on a feminine plural noun is a giveaway.