I did some thinking and I found a pattern for stem-changing spanish verbs. It’s sort of complicated so be sure you understand a few things. Keep in mind that the letter a and the letter i cannot be a stem-changing vowel.
You need to know soft and hard consonants (in Spanish).
Hard consonants in Spanish: b, c (when like a k), d, g, gu, k, n, qu, t, v
Soft consonants in Spanish: c (when like an s), f, g (when like an h), j, m, p, s, z
The other consonants are neither. Now that you know these, we can get to the pattern.
- Stem changer vowels are always the second vowel space from the right.
- If that second vowel from the right is to the right of a soft consonant, it changes. Examples: jugar, poder, tener
- If that vowel is to the right of a hard consonant, and that consonant is to the right of a vowel, then it changes. Example: divertir, acostar
- If there is an o after a hard-sounding c, and before 2 consonants in a row, it changes. Examples: costar, acostar
- It always changes if it is after a v or a qu. If it doesn’t, it’s an irregular verb. Examples: venir, vestir, volver, querer
Thank you for reading. I hope this helped.