Two different things share the name "superlative"
English speakers usually meet "superlative" as one category: add -est or put "most" in front, done. Spanish grammar splits that single English idea into two constructions that don't work the same way and aren't interchangeable. The RAE's own glossary defines them separately: the superlativo relativo expresses a property held by one member of a group to a higher degree than the rest of that group, while the superlativo absoluto expresses the extreme degree of a quality without comparing it to anything.
Ana es la más alta de la clase ("Ana is the tallest in the class") is relative: there's a group (la clase) and Ana is being ranked against it. Ana es altísima ("Ana is extremely tall") is absolute: there's no group, no ranking, just a claim about how tall Ana is on her own. Swap them and the sentence still parses, but it stops meaning what you meant: la más alta can leave its group implicit in context, and altísima de la clase isn't how the absolute form is used at all. It's the same kind of collapse-in-English problem that makes por and para hard to tell apart: one English word, two unrelated Spanish grammar rules underneath it.
The relative superlative: article + más/menos, group optional
The RAE's Nueva gramática básica breaks the relative superlative into three parts: the first term (the entity being described, Ana), the quantificative group (más or menos, or one of the irregular forms below), and a restrictive complement or "coda" that names the comparison group: de la clase, de mis amigos, del equipo. That third part is explicitly optional in the RAE's own account: la más alta is a grammatical expression even with the group left unstated if context already supplies it.
A common noun-plus-adjective pattern is la película más interesante de la temporada. The RAE also shows irregular superlatives before the noun, as in la peor película del certamen; the construction does not impose one fixed position on every adjective.
Irregular superlatives and their regular alternatives
Bueno, malo, grande and pequeño have the irregular comparative forms mejor, peor, mayor and menor. Regular alternatives also exist: the RAE accepts más bueno and más malo for character or taste, and más grande and más pequeño for size. Meaning decides which form fits; the regular alternatives are not blanket errors.
Mayor and menor carry a wrinkle the -ísimo forms don't: in everyday use they lean toward age rather than physical size. Mi hermano mayor means "my older brother," and a Spanish speaker describing a bigger house will usually reach for una casa más grande rather than una casa mayor, even though mayor is available and grammatically fine for size too. The RAE itself doesn't restrict mayor/menor to age, but the age reading is the one that dominates in practice, which is why más grande and más pequeño survive in casual speech alongside the irregular forms.
Forming -ísimo: the spelling changes exist to protect the sound
For most adjectives, forming the absolute superlative just means dropping the final vowel and adding -ísimo, -ísima, -ísimos, or -ísimas, with the accent on that í mandatory: alto becomes altísimo, lenta becomes lentísima. Adjectives already ending in a consonant like -l or -r just take the suffix directly (fácil becomes facilísimo, popular becomes popularísimo), per the RAE's own list in El buen uso del español. Like demonstrative adjectives, the resulting form still has to agree in gender and number with the noun it modifies: altísimo, altísima, altísimos, altísimas, not one fixed shape.
Several endings force a spelling change to keep the pronunciation intact rather than the grammar. A final -z becomes -c- before the suffix (feliz → felicísimo, RAE's own example), a final -co becomes -quísimo (rico → riquísimo) and a final -go becomes -guísimo (largo → larguísimo), both there so the hard c/g sound survives in front of í. Some adjectives insert -c-: mayor becomes mayorcísimo, and joven becomes jovencísimo. This is not a general rule for every -or adjective, as popularísimo shows. That means an already-irregular comparative like mayor can still take its own absolute superlative, a form most Spanish courses skip entirely.
The vowel-ending rules split by stress. Adjectives ending in the unstressed diphthong -io/-ia drop both vowels: sucio becomes sucísimo, amplia becomes amplísima. Adjectives ending in the stressed hiato -ío/-ía drop only the final vowel, keeping the accented í: frío becomes friísimo. The two look almost identical on the page (sucio vs. frío) but land on different suffix shapes because Spanish treats a diphthong and a hiatus as different vowel structures.
The Latin fossils: -érrimo, and why "acérrimo" doesn't mean "acre" anymore
A small closed set of adjectives keeps a superlative suffix inherited directly from Latin, -érrimo/-érrima, not the regular -ísimo. The RAE's Nueva gramática lists it alongside -ísimo as one of the two suffixes that mark "grado extremo," but the list of words that still use it is short and mostly frozen: pobre → paupérrimo, célebre → celebérrimo, íntegro → integérrimo, áspero → aspérrimo, and acre → acérrimo.
Acérrimo appears in phrases such as enemigo acérrimo, "bitter" or "sworn enemy," and defensor acérrimo, "staunch defender." Acre on its own means acrid or bitter-tasting, but most speakers who say enemigo acérrimo aren't thinking about taste at all. In these uses it expresses a strong degree of commitment or opposition; literal taste is not the intended sense. Spanish speakers use it fluently without necessarily being able to name the base adjective it comes from, which is a different kind of irregularity than mejor/peor: those are irregular but transparent, while -érrimo forms are irregular and largely opaque outside the handful of phrases that kept them alive.
A third option the RAE names but most courses skip: prefixes
The RAE's own account of the absolute superlative doesn't stop at suffixes. Its glossary on grado superlativo explicitly lists prefixation as a third way to build one, giving reguapo, superatractivo, and hipercalórico as examples alongside altísimo: re-, super-, mega-, and hiper- attached directly to the adjective, no suffix needed.
That prefix strategy isn't evenly distributed across the Spanish-speaking world. A 2019 study of intensifiers in Buenos Aires and Tucumán, Argentina describes re- as occupying a specialized, highly productive slot in Rioplatense Spanish specifically, well beyond its more occasional use elsewhere: relindo, rebueno, and recaro are ordinary spoken Spanish in Argentina and Uruguay in contexts where riquísimo or buenísimo would sound equally natural but noticeably more formal. The prefix chain can also stack for extra emphasis in casual speech: recontra- and requete- both function as intensified versions of re- itself. Re- and its relatives are associated with informal registers and also appear in informal writing; -ísimo is used across both informal and formal registers.