Answer:
When conjugating infinitive verbs in the preterite tense, there are 3 AR-type infinitives that change their spelling in the first person singular (Yo) conjugation.
Specifically, AR infinitives that end in CAR, GAR or ZAR have spelling (or orthographic) changes in the "Yo" conjugation only.
Why? In each case, the spelling change serves the purpose of maintaining the pronunciation in the conjugated verb that closely resembles the pronunciation of the original infinitive.