I'm not completely sure on the grammar usage of と、～た, in these following sentences, but have been given the general explanation that the imply an unexpected result following a normal (one time?) action.
１．弟は昼ごはんを食べると、片付けもせずに出かけてしまった。
２．私が「ごめん」と言うと、友達は「あやまるのはぼくのほうだ」と言った。
I'm also unsure of how to translate と as it applies to the first half of the sentence in these cases either, or the tense.
「と」は接続助詞といいます。
It is a connective particle.
１．弟は昼ごはんを食べると、片付けもせずに出かけてしまった。
２．私が「ごめん」と言うと、友達は「あやまるのはぼくのほうだ」と言った。
In these phrases, the connective particle "shows at the opportunity when a certain situation happens or an opportunity to notice it."
When my younger brother finished eating lunch, he has gone out without clearing the table.
When I said "I'm sorry," my friend said "It's me that should apologize to you."
