The context sentence is 東方の空が明るくなりだした
I understand it just fine except for the conjugation of what I am assuming is “なる” (to become). I tried to look it up on japaneseverbconjugator.com, but from the sentence I am assuming it is meant to be the past indicative, which this website claims to be ** なりました**, instead of なりだした. I am just starting out to learn verb conjugations, so there is probably something very easy that I haven’t learned yet. Could someone be so kind as to explain to me why ま becomes だ, or whether this is a different verb form alltogether?