Change the vowel inside the stem of certain verbs when they are in the appropriate form.
As with other present tense issues, you will use this to talk or write about what is happening now or in the near future.
The first thing to remember is that nothing about the endings is any different than with the other regular present tense verbs. That part of conjugating stem changing verbs is the same.
The difference is in the stem (what is left after you drop the AR, ER or IR). For the subjects "nosotros" and "vosotros," the stem is exactly what you expect, merely drop the AR, ER or IR. The other forms, however, are a little more problematic. The first step involves identifying which of 3 kinds of changes it will make: there is "E" to "IE", "O" to "UE" and "E" to "I". The only way to know which change it makes, or even if it makes a change at all, is to keep that in mind when you first learn the verb. Most text books identify stem changers by listing the change in parentheses after the verb, like poder (ue) and querer(ie) .
A common question for verbs like preferir and encontrar that have more than one vowel in the stem is: Which vowel changes? Once you find the stem, it is always the final vowel in that stem that will change. So in preferir the second "E" will change to "IE" and in encontrar the "O" will change to "UE".
In the above interaction, you can see examples of the different kinds of stem changing verbs. After selecting a verb, you will see both the unchanged and the changed stem. Then click on a subject to finish the conjugation. You can also click on the "All Forms" button to see all six forms at once. From that view you can see why some instructors like to refer to these verbs as "boot" verbs, because the forms inside the "boot" are the ones that have the change in their stem.
On a final note, you will see that there is one verb, jugar, that is listed as a "U" to "UE" and you may wonder why we say there are only 3 kinds of changes. This is the only verb in that category, so it is usually just lumped in with the "O" to "UE" stem changers.
There are other tenses and moods where many of these same stem changers have behave in a similar manner. So knowing what verbs have a stem change and what those changes are will learning those future changes a little easier.
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