If the aorist stem is different from the present stem (i.e. the verb is irregular) then the aorist stem is used, but the present endings are used. For example, the verb [[ερχομαι]] (I come) is irregular, with the aorist principle part as [[ηλθον]] (I came). Hence this is a second aorist verb, and the ''aorist, active, nominative, masculine, singular'' form is [[ελθων]] (not ελθσας - which doesn't exist). Note that the augment of '''ηλθον'''' is removed in '''ελθων'''.