Confused with one of the context sentences for "eastward"(東方)

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, 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?


It’s a compound verb, なる + 出す- as an auxiliary verb, 出す means “to begin (verbing)”.


Especially with a nuance of “suddenness” or “bursting out”.

Another “to begin (verbing)” auxiliary is はじめる, but it’s more neutral.

Grammatically, you could say なきはじめる for “to start crying” なく + はじめる, but なきだす, with the nuance of suddenness, is more commonly seen.