Below is a passage illustrating the way in which the main vowel of the verb is diphthongized when conjugated in the present tense, both indicative and subjunctive moods. Note that some verbs like pedir also change their main vowel, but this is not diphthongization.

1. –Te lo cuento preocupado; no te miento: últimamente duermo mal y aunque me acuesto cansado no puedo conciliar el sueño, me siento nervioso; así que, si digo algo ofensivo, te recomiendo no me lo tomes en cuenta; además, te ruego que me corrijas si hago algo mal.

2. –Cariño, no te pierdas en disquisiciones (irrelevancies). No me pidas a estas alturas (at this time) que te comprenda. Recuerdo que me pareciste tonto desde el primer momento, aunque eso no ha impedido que te quiera. De paso, te recomiendo que no riegues las flores de plástico, te pido que no lleves el paraguas abierto por el comedor aunque llueva en el jardín y que no vuelvas a meter al gato en el microondas.

Forms of radical changing verbs

There is another very large group of verbs which Spanish-speakers call “irregular,” but which English- speakers refer to as “radical/stem changing” verbs. The French are tormented by this group, frequently calling them “régulièrement irréguliers!” By far the most helpful of these designations is the English one, for most of the verbs in question follow a set pattern, and once the pattern is learnt, you have an easy and accurate access to a wide range of verbs.