Diphthongation is very common in Spanish. It concerns mainly verbs, but also nouns, adjectives and adverbs. It consists in replacing the tonic vowel of a word by a double vowel pronounced in the same syllable.

Spanish diphthongation

Before tackling verb diphthongation (the most difficult part…), let’s see how other words behave when they diphthongate: nouns, adjectives and adverbs.

Nouns

Many Spanish nouns whose root vowel on the penultimate syllable under the tonic accent was originally e or o have undergone diphthongation, and are now written with ie and ue :

• puerta, sueldo, fuerza, cuerda, recuerdo
  door, salary (pay), strength, rope, memory (record);
• hierba (ou yerba), hierro, infierno, tierra
  grass, iron, hell, earth.

Adjectives and adverbs

This transformation is found in some adjectives and adverbs:

• caliente, bueno, fuerte, fuertemente, cierto, ciertamente
  hot, good, strong, strongly, true (certain), certainly.​

Diphthong and hiatus

Both are very common in Spanish, and easy to distinguish: the hiatus adds a syllable to the word, as in the following examples, while the diphthong does not:

• parque (2 syllables : par-que) –> parquear (3 syllables : par-que-ar)
  park                 to park ;

• aire (2 syllables : ai-re) –> aéreo (4 syllables : a-é-re-o)
  air              airborne

Diphthong verbs

What is a diphthong verb in Spanish? In the present indicative, but also in the subjunctive and imperative, some Spanish verbs diphthong when the vowel of the stem on the penultimate syllable is e or o. This results in ie and ue under the tonic accent.

• Entender: entiendo, entiendes, entiende, entendemos, entendéis, entienden.
  Understand: I understand, you understand, he understands, we understand, you understand, they understand.
• Poder: puedo, puedes, puede, podemos, podéis, pueden.
  Can: I can, you can, he can, we can, you can, they can.

Diphthongation is frequent, but not systematic, many verbs do not diphthongate, as in the following examples.

• Comer: como, comes, come…
  To eat: I eat, you eat, he eats…
• Aportar: aporto, aportas aporta…
  To contribute: I contribute, you contribute, he contributes…
• Rezar: rezo, rezas, reza…
  To pray: I pray, you pray, he prays…

Unfortunately, there is nothing to distinguish them a priori. Only by learning and practicing can you know if a verb has a diphthong or not.

A single verb in u diphthong. Since it is very common, it is useful and easy to remember.

• Jugar: juego, juegues, juegue, jugamos, jugáis, jueguan.
  To play: I play, you play, he plays, we play, you play, they play.

Verbs that diphthongize do so in the present tense of all modes (indicative, subjunctive and imperative), according to the same phonetic and orthographic transformation.

Subjunctive tense

• Que entienda, que entiendas, que entienda, que entiendan.
  That I understand, that you understand, that he understands, that they understand.
• Te lo digo para que te acuerdes.
  I’m telling you so that you remember.

Imperative

• Aprieta, apriete, aprieten, apriétalo.
  Squeeze, squeeze, squeeze it.
• ¡Entiéndeme bien!
  Understand me well!

Irregular verbs in with an ending in go in the first person singular do not diphthong in that person.

• Venir: vengo, vienes…
  To come: I come, you come…
• Tener: tengo, tienes…
  Have: I have, you have…

Spelling changes imposed by diphthonging verbs

When the root of a verb begins with the vowel that diphthongs, it undergoes a spelling change.

• Oler: huelo, hueles, huele, olemos, oléis, huelen, que no huelan.
  To smell: I smell, you smell, he smells, we smell, you smell, they smell, they do not smell.
• Errar: yerro, yerras, yerra, erramos, erráis, yerran, ¡no yerres!
  To make a mistake: I make a mistake, you make a mistake, he makes a mistake, we make a mistake, you make a mistake, they make a mistake, don’t make a mistake!

Several pronunciations and spellings are sometimes allowed.

• Erguir: yergo/irgo, yergues/irgues, yergue/irgue, yerguen/irguen.
  Erect: I erect, you erect, they erect.

If the consonant preceding the o that undergoes diphthongation is a g, an umlaut specifies that the following u must be pronounced. Otherwise, it would be silent, as in guerra.

• Degollar: degüello, degüellas, no lo degüelles, que no lo degüellen.
  To slaughter: I slaughter, you slaughter, do not slaughter, let them not slaughter.
• Regoldar: regüeldo, regüeldas, que no regüelden, ¡no regüeldes!
  To burp: I burp, you burp, don’t burp, don’t burp!
• Avergonzarse: me avergüenzo, te avergüenzas, que se avergüence, avergüéncense.
  To be ashamed: I am ashamed, you are ashamed, let him be ashamed, be ashamed.​

Verbs and other related words

The link between the diphthongation of a verb and that of other words built on the same root is not systematic, as the examples below show.

• Arrendar: arriendo; el arriendo.
  To rent: I rent; the rent.
• Encontrar: encuentro; un encuentro.
  To meet: I meet; a meeting.
• Calentar: caliento; caliente.
  To heat: I heat; hot.

• Costar: cuesto; el costo.
  To cost: I cost; the cost.
• Ascender: asciendo; el ascenso.
  To ascend: I ascend; the ascent; the promotion.

• Portar: porto; el puerto, la puerta.
  To carry: I carry; the port (harbour), the door.

Here again, only learning and use will help you find your way around.

Diphthongation is therefore very common in Spanish. It can be confusing at the beginning of the learning process, but it is not so difficult to master, because it almost always follows the same rules. 

The most important thing is to know when it occurs, without making mistakes. With practice, it becomes automatic and you don’t think about it anymore.

👉 Practice now!

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