I thought your name was Ilaria, but if you want to be Certain today, that's up to you.
because "Today they are certain" would be: "Oggi sono certi/certe" (male/female plural)
yes, but with a form of "essere" you have to use an adjective and not an adverb.
The fact is that the word «certo» (and all of its different versions to match gender and number) is an adjective because it agrees with the subject (which can be omitted in Italian because of conjugated verbs). In this sentence, the subject must be «io» because the conjugation is «sono» and the adjective «certo» (masculine singular); so the speaker of this sentence (despite robot voice lady) is one male. If «certo» were an adverb (which it is not), it would not have all of those endings that sandrabruck mentioned. The corresponding adverb is «certamente», and «-mente» is like the "-ly" in English, a common adverb suffix.
Why is certo here used as an adjective when we are dealing with adverbs? Just saying.