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How do you tell when a Spanish verb changes its vowel?

So far, we've seen that certain otherwise regular Spanish -ar verbs change the vowel of their stem when that vowel is stressed. There are essentially two possible vowel changes for -ar verbs: e > ie, as in pensar; and o > ue, as in encontrar.

We already mentioned that we can't generally tell just from an infinitive whether or not the verb will change its vowel. So what can we do to help us tell whether or not a given verb has a vowel change?

Eliminating vowels that can't change

Firstly, it's worth just reminding ourselves exactly when are the possible vowels that can and can't change:

In other words, if it's an -ar verb and the last vowel before the -ar is a, i or u (and it's not jugar), then you're home and dry: there'll be no vowel change.

So what about the other verbs that could potentially change? Well, as we mentioned, there's no sure-fire way to tell just from the infinitive. But we can generally tell if we consider the verb and its related vocabulary.

Looking at related nouns and adjectives

For a given verb, there's often a related adjective or noun made from the same root. These can give us a clue as to whether or not the verb changes its vowel:

By related, we mean "based on the same root". So for example, on the basis of the word caliente (hot), we predict that calentar (to heat) will have a vowel change. The vowel change is always the same as in the related word (and in any case is predictable: if it changes, e always changes to ie and o always changes to ue).

The main "gotcha" to be aware of with this rule of thumb is that:

For example, we might at first sight think that manifestar doesn't change its vowel, because the vowel doesn't change in manifestación. Or we might think that pensar doesn't change, because there's no change in pensamiento. But what has happened here is we've "chosen the wrong word": we need to find the related word with the same number of syllables as the infinitive (or strictly speaking, the word where the changing vowel is stressed). When we look at manifiesto and pienso (now a literary word not used in everyday Spanish), we indeed see the vowel change.

Nonetheless, there are a good few pairs that, if we learn them together, will help us remember the vowel change:

Note that the related words without the vowel change have extra syllables so that the vowel isn't stressed. In other words, the rule for the related noun/adjective is essentially the same as for the verb: forms where the changing vowel is stressed have the changed version.

In generall, compounds of words formed with prefixes follow the same vowel change as their base word. So for example, comprobar and desaprobar follow the same pattern as probar. Occasionally, we can use this to "work backwards". There's no word related to negar (to deny) that will help us tell if it has a vowel change, but based on renegar, we can predict that it does. And occasionally, we need to be cautious. For example, conjugar (to combine, conjugate) is totally regular and does not follow the pattern of jugar! Similarly, we could predict, on the basis of nuevo (new), that renovar (to renew, renovate) will change its vowel.

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