Questions & Answers about Теперь мне ясно, почему ты опоздал.
Теперь means now. In this sentence, it sets up the idea now it is clear to me.
Starting with Теперь is very natural because it contrasts the present moment with an earlier state when the speaker did not understand. English often does the same:
Now it’s clear to me why you were late.
You could move теперь to a different position, but putting it first is the most neutral and common here.
Because Russian expresses this idea as it is clear to me, not I am clear.
So:
- мне = to me / for me
- ясно = clear
Literally, Мне ясно is something like To me, it is clear.
English uses I understand or it’s clear to me, but Russian often prefers this impersonal structure.