When to Change ‘Y’ to ‘E’ and ‘O’ to ‘U’ in Spanish

Two of the most common conjunctions in Spanish — y (meaning "and") and o (meaning "or") — can change spelling and pronunciation based on the word that follows. In that way, they are much like the "a" of English changing to "an" before a vowel sound. And, like the "a"-to-"an" change, the transformation is based on how the following word is pronounced rather than how it is spelled.

When Do Y and O Change?

Both the y and o changes help keep the conjunction from blending into the following word. (The blending of two words into what sounds like one is called elision when it involves the dropping or omission of sounds, and it is common in both English and Spanish.)

Here are the changes that are made:

• Y becomes e when it precedes a word that begins with the i sound. Typically, y becomes e when it precedes most words that begin with i- or hi-.
• O becomes u when it precedes a word that begins with the o sound. Thus o becomes u when it precedes words words starting with o- or ho-.

Because the changes are based on pronunciation rather than spelling, y does not change before words, such as hierba, that begin with the ia, ie, io, or iu sound, regardless of spelling. Those two-letter combinations are known as diphthongs; the initial sounds are very similar to the Spanish "y" sound when "y" comes before a vowel.

Sample Sentences Showing Use of Y and O

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Nuestro conocimiento nos enseña dos cosas claras: posibilidades e imposibilidades. (Our knowledge teaches us two clear things: possibilities and impossibilities. The e is used because imposibilidades begins with the i sound.)

Fabricamos barras e hilos de cobre. (We manufacture copper bars and wires. The e is used because hilos begins with the i sound even though the first letter is h.) 

Está enteramente construido de nieve y hielo. (It is built completely of snow and ice. The y does not change because hielo begins with the ie diphthong.)

Hay un equilibrio osmótico y iónico. (There is an osmotic and ionic equilibrium. The y is used because iónico begins with the io diphthong.)

Hay muchas diferencias entre catolicismo e hinduismo. (There are many differences between Catholicism and Hinduism. The y changes to e because hinduismo begins with the i sound even though its first letter is h.)

Vendemos productos de limpieza e higiene. (We sell cleaning and hygiene products. Higiene begins with the i sound.)

Usamos punto y coma para separar las frases u oraciones que constituyen una enumeración. (We use a semicolon to separate phrases or sentences that make up a list.)

No recuerdo si fue ayer u hoy. (I don't remember if it was yesterday or today. Unlike with the changes involving y to e, the o changes even though oy is a diphthong.)

¿Qué operador de teléfonos ofrece las tarifas más baratas para viajar a África u Oriente Medio? (Which phone operator offers the lowest costs for traveling to Africa or the Middle East? The rule of changing o to u applies even if the word following is a proper noun.)

La Can Make Similar Change

The desire to keep the sounds of important words from being lost due to elision is also behind the changing of la to el in some circumstances with feminine sounds. Although there are exceptions, el is used instead of la before singular feminine nouns where the first syllable of the noun is stressed. Thus "the eagle" is el águila even though águila is feminine. The change doesn't occur with plural nouns or where the stress isn't on the first syllable. In standard written Spanish, una becomes un (meaning "one," "a," or "and") under the same circumstances. Thus, "an eagle" is un águila.

These changes and those involving y and o are the only situations where Spanish changes words depend on sounds that follow.

Key Takeaways

• The Spanish conjugation y (meaning "and") changes to e when the word that follows begins with the i sound.
• The Spanish conjugation o (meaning "or") changes to u when the word that follows begins with the o sound.
• These changes are triggered by pronunciation only, not how a word is spelled.