A stem is the part of the verb that comes before the ending (which, in infinitive verbs, is always either ar, er, or ir). In regular conjugations, the stem doesn’t change, where in irregular conjugations, it can be completely different. In stem-changing verbs, the change is in the last vowel of the stem.
Stem-changing verbs in Spanish can also be thought of as 1, 2, 3, 6 verbs. This is because in the traditional conjugation tables, the conjugations that change are the first, second, third and sixth. It might be easier to think of it as the ones that are not affected as there are fewer: just nosotros and vosotros. In case you were wondering about vos, it is also unaffected. These are conjugated just as they are when it’s a regular verb.
There are a few different ways that stem-changing verbs in Spanish can change. The key is in the vowels in the verb. Let’s see the different possibilities:
e -> ie (e.g: pensar)
e -> i (eg: conseguir)
o -> ue (eg: encontrar)
others (i -> ie, u -> ue)
hope this helps :)