if day or morning dawns, it begins:
The morning dawned fresh and clear after the storm.
if a period of time or situation dawns, it begins:
The age of Darwin had dawned.
if a feeling or idea dawns, you have it for the first time:
It began to dawn that something was wrong.
dawn on somebodyphrasal verb
The ghastly truth dawned on me.
It dawned on me that Jo had been right all along.