Learning Spanish verbs can be grouped into four groups: regular, homophones (sound the same), the straight-up irregular verbs that you'll need to memorize, and then groups with just slight stem changes. It is this last group that we'll talk about today.
First things first, these verbs are mostly only irregular in three tenses: present, subjunctive, and imperative, with a few exceptions we'll see later. Within these, it's only for the yo, tú, él / ella / usted, and ellos / ellas / ustedes forms. That's it!
These verb groups are characterized by having a particular vowel accented (as in the emphasis is on that syllable), and you then change that vowel somehow. This is way too abstract, so let's just list them.
|Accented vowel||Changed to||Sample verb|
|o (ir verbs)||ue / u||dormir|
|e (ir verbs)||ie / i||sentir|
|e (ir verbs)||i||pedir|
So these are straight-forward, except the last two. It's the same rule! Unfortunately, sometimes verbs with a stressed e that end in -ir are changed into ie or i, while others are just changed into i. You'll just have to memorize them.
Let's take these groups one-by-one and look at how they are conjugated.
These verbs have the emphasis fall on the e syllable, and end in -ar or -er. Here is a long, but not exhaustive list: alentar, atender, calentar, comenzar, confesar, defender, descender, despertar, empezar, encender, mentar, pensar, perder, quebar, sentar.
Can you detect the pattern? Make sure you can see it, because once you can recognize it, the conjugation while is quite simple: just replace the e with an ie in all three singular forms and 3rd person plural. Then conjugate like normal.
Here's a table for empezar (to begin / to start):
|él / ella / Ud.||empieza|
|ellos / ellas / Uds.||empiezan|
Pretty easy, right? As long as you can detect when this rule is activated.
This next grouping is almost identical to the previous, but the emphasized vowel is an o, and it is changed to a ue. Verbs in this group include acordar, acostar, contar, costar, mostrar, recordar, devolver, doler, llover, morder, mover, volver.
The irregular tenses and pronouns are exactly the same as the previous section, but again, we're changing the o to a ue. Here is a table for mostrar, "to show":
|él / ella / Ud.||muestra|
|ellos / ellas / Uds.||muestran|
Now remember these are just -ar and -er verbs. Now we'll handle -ir verbs.
Now these verbs are a little confusing. In the tenses covered by previous verbs, they are conjugated exactly the same as other accented o verbs, by changing the o to ue.
However, in all imperfect subjunctive forms as well as 3rd person preterite forms, the o is changed into a u. Fortunately, the main verbs in this category are just dormir and morir.
Let's look at dormir, "to sleep". For present, present subjunctive, and imperative, it is exactly as the previous section. So I will only show the different ones, preterite and imperfect subjunctive. First preterite:
|él / ella / Ud.||durmió|
|ellos / ellas / Uds.||durmieron|
Notice the change in both 3rd person forms to a u. Imperfect subjunctive:
|él / ella / Ud.||durmiera|
|ellos / ellas / Uds.||durmieran|
The endings are exactly the same, we just swap out the o for a u.
These verbs are exactly like -o_ir verbs, but instead of swapping o with u, we will change the e into a i in 3rd person preterite conjugations and all imperfect subjunctive conjugations.
I won't show the conjugations, but some sample verbs are consentir, digerir, divertir, hervir, invertir, mentir, preferir, referir, and sentir.
Wait, isn't this exactly the same as the previous section?! Unfortunately, yes. There are some verbs that match this pattern where we do not substitute the e for ie, but rather for an i, even in present, present subjunctive, and imperative tenses.
We'll just have to memorize the fortunately small list that triggers this exception: pedir, impedir, henchir, expedir, and servir.
Here is a sample conjugation for present tense henchir (to fill up):
|él / ella / Ud.||hinche|
|ellos / ellas / Uds.||hinchen|
Again, note the change of e to i in all but nosotros and vosotros forms.
In some ways, these small stem-changing verbs are the most challenging, and can be harder to remember and recognize that those which are radically different. But if we break them down into groups and manageable pieces, we can master them in no time at all, which is really the entire idea behind this site.