Although some verbs follow the pattern you have learned for regular present tense verbs, they contain certain spelling changes that must be memorized.  For example, the verbs conocer, recoger, and actuar all contain spelling-changes:  conozco, recojo, and actúo.

Verbs usually change spelling in order to maintain the pronunciation that exists in the infinitive.  For example, recoger changes to recojo in the yo form to keep pronunciation consistent.  Interestingly, many spelling changes appear in the first person singular of the present indicative.

Last, most spelling change verbs are highly affected in the present subjunctive because its stem is taken from the yo form of the present indicative: