Verbs with an "-a", "-e", or "-o" as the last letter of the stem, after the infinitive ending is dropped, need an added accent mark on their past participle form, but only if the original verb ends in...Verbs with an "-a", "-e", or "-o" as the last letter of the stem, after the infinitive ending is dropped, need an added accent mark on their past participle form, but only if the original verb ends in "-er" or "-ir". The accent mark must be placed on the "i" of "-ído". For example, "aburrido" is a past participle that derives from the verb "aburrir". When past participles are used as adjectives, the ending "o" must change to agree with the noun they describe.
