Answer: Spanish verbs that end in -GAR must change the g to a gu before adding the ending of "yo" form in the preterite tense.
"Regar" goes to "yo regué"
"Rogar" goes to "yo rogué"
"Vagar" goes to "yo vagué"
"Tragar" goes to "yo tragué"
"Fregar" goes to "yo fregué"