The truth finally dawned on her.
She finally realized/understood the truth.
Student or Learner
The truth finally dawned on her
what dawned on means in this sentence
does it means to realize
Yes - but it usually means to realize gradually, and often after a long time; think of the gradually growing light at daybreak (at least, it was gradually growing in the part of the world where the image was first used, and before light pollution interfered with the effect .) (Actually, 'realize' puts it the wrong way round: if A dawns on B, B realizes A.)
at first i didnt believe her but finallly i realize that she was telling the true or truth
how can i fit dawn on in this sentence
You might find this useful in other contexts. But for this one, stick to Mike's.