So much for the general rule for building the subjuntivo presente of regular verbs.
You also should take a look at some exceptions:

Building the subjuntivo presente – exceptions to the general rule:

1. “o” shifts to “ue”
   
   💡 For example:
   contar (to tell)
   cuente, cuentes, cuente, contemos, contéis, cuenten

2. “o” turns into “ue” + “o” changes into “u”
   
   💡 For example:
   dormir (to sleep)
   duerma, duermas, duerma, durmamos, durmáis, duerman

3. “e” turns into “ie”
   
   💡 For example:
   pensar (to think)
   piense, pienses, piense, pensamos, penséis, piensen

4. “e” changes to “i”
   
   💡 For example:
   pedir (to ask for)
   pida, pidas, pida, pidamos, pidáis, pidan

5. “e” tuns into “ie” or “i”
   💡 For example:
   sentir (to feel)
   sienta, sientas, sienta, sintamos, sintáis, sientan

6. Verbs ending in “-uir“ additionally add “y”
   💡 For example:
   concluir (to finish)
   concluyo, concluya, concluyas, concluyamos, concluyais, concluyan

7. Further exceptions:
   Verbs ending in -acer, -ecer, -ocer or -ucir, form their subjuntivo with -zco.
   💡 For example:
   conocer (to know)
   conozca, conozcas, conozca, conozcamos, conozcáis, conozcan