The one rule: tan before an adjective or adverb, tanto everywhere else
Look at the next word. If it is an adjective or an adverb, the short form: tan alto, tan rápido, tan bien, tan lejos. If it is a noun, tanto agrees with it like any determiner: tanto dinero, tanta gente, tantos libros, tantas cosas. If nothing follows — the word closes the clause after a verb — the long form stays: trabaja tanto, no comas tanto. The academies' Diccionario panhispánico de dudas puts it in one sentence: as a determiner tanto agrees in gender and number (cada tantos pasos, tantas puertas); as an adverb it is invariable and takes the full form tanto when it modifies a verb («un lugar que tanto odio») and the short form tan before adjectives and adverbs («frases tan inteligentes», «tan lejos de la ciudad»).
The English words do not help, because so and as never change shape: so tall, so much, as tall as, as much as. Spanish shortens the word in one place and makes it agree in another, and the test is always the same — what comes next.
Every shape, in one chart
The last four rows are the exceptions, and they are the whole of the exceptions: un tanto keeps its shape because it is a fixed phrase meaning a little, and más, menos, mayor, menor, mejor and peor keep the long form in front of them because they are comparatives already — tanto más, tanto mejor. The one twist is mayor meaning elderly rather than greater, which does take tan: the academies' example is «Nunca había estado con un hombre tan mayor».
| English | Spanish | Why |
|---|---|---|
| so tall | tan alto | an adjective follows |
| so fast | tan rápido | an adverb follows |
| so well | tan bien | an adverb follows |
| so much money | tanto dinero | a noun, masculine singular |
| so many people | tanta gente | a noun, feminine singular |
| so many books | tantos libros | a noun, masculine plural |
| he works so much | trabaja tanto | after a verb, nothing follows |
| not so much | no tanto | nothing follows |
| a little strange | un tanto raro | un tanto = a little, never shortened |
| all the more | tanto más | the long form before más, menos, mejor, peor |
| so much the better | tanto mejor | the long form before a comparative |
| she is so old | es tan mayor | mayor meaning old, not bigger, takes tan |
As…as: tan…como and tanto…como
The equality comparison keeps the rule and adds como after it. Tan + adjective or adverb + como: mi hermano es tan alto como yo, corre tan rápido como su padre — the academies' example is «se esfumó tan sigilosamente como había llegado». Tanto, agreeing, + noun + como: tengo tantos libros como tú, «hay casi tantos curas como prostitutas». Verb + tanto como: trabaja tanto como yo. The consequence works the same way with que: estaba tan cansada que se durmió, «había pasado tanta hambre que había perdido de vista toda proporción» — and there is no comma before that que.
There is an older correlative with cuanto — tantas personas cuantas se elijan — that the academies record and then note is normally como today. You will meet it in print; you do not need it in your mouth.
The both-and tanto that never changes
Tanto … como also makes a conjunction, both … and, and in that job it never agrees and never shortens: tanto hombres como mujeres means both men and women, tanto masculinas como femeninas means masculine and feminine ones alike. Change the ending and the meaning changes with it — tantos hombres como mujeres is as many men as women, a comparison of number. The academies set the two side by side for exactly this reason. One consequence: a subject joined by both-and tanto takes a plural verb — «Tanto yo como mi hijo se lo agradecemos».
The three mistakes English speakers make
Tan mucho — English says so much, and much looks like an adjective, so the short form gets put in front of it. But so much is one word in Spanish: tanto. Trabaja tanto, no tanto, tanto dinero. Tanto bueno, tanto rápido — the opposite slip: an adjective or an adverb takes tan, always. Tanto gente — the agreement: gente is feminine, so tanta gente; the noun decides the ending the way it does after every determiner.
If muy and mucho are already automatic for you, you have this rule already: tan is to tanto what muy is to mucho. Muy alto and tan alto, mucho dinero and tanto dinero, come mucho and come tanto — the same split, one step up in strength.
Why do English speakers mix them up?
Because nothing in English warns you that the word changes. So and as do one job each and never move — so tall, so much, as tall as, as much as — while Spanish asks you to look ahead at the next word before you choose the shape. The slips cluster at exactly the two seams: so much (tanto, not tan mucho) and so + adjective (tan, not tanto). Read the chart once with the third column covered and the rule is usually yours in an afternoon; what takes longer is doing it at speed, which is what the quiz measures.
There is no first-party error rate on this page yet: our mistake tagger counts ser vs estar, por vs para, the tenses and the object pronouns, not tan and tanto. When it does, the number will sit here, the way it does on the sibling pages. Until then the quiz is the measure.
What comes after tan and tanto?
Muy vs mucho is the same split with very and a lot, one step down in strength, and the place most people learn the pattern first. Bien vs bueno decides whether tan meets an adverb or an adjective — tan bien, tan bueno — and qué vs cuál is the next small word English translates too literally. The free Spanish level test places you on the A1–C2 scale in five minutes.