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.

The dative is used because the sentence describes a state experienced by someone.

In Russian, words like ясно, понятно, интересно, трудно, холодно often combine with the dative:

• Мне ясно = It’s clear to me
• Ему понятно = It’s understandable to him
• Нам трудно = It’s hard for us
• Тебе холодно = You are cold

So мне is dative because the speaker is the person to whom the situation is clear.

Here ясно is not a normal adjective agreeing with a noun. It is a predicative word (sometimes explained to learners as a short-form neuter adjective used impersonally).

It means clear in an impersonal construction:

• Мне ясно = It is clear to me

Compare:

• ясный день = a clear day
  Here ясный is a regular adjective modifying day.
• Мне ясно = It is clear to me
  Here ясно stands by itself as the predicate.

A very common synonym in this kind of sentence is понятно:

• Теперь мне понятно, почему ты опоздал.

Because почему ты опоздал is a subordinate clause.

Russian uses a comma before clauses introduced by words like:

• что = that
• почему = why
• когда = when
• если = if
• потому что = because

So:

• Теперь мне ясно, почему ты опоздал.

The comma is required in standard writing.

Here почему means why.

So:

• почему ты опоздал = why you were late

It introduces the reason as a question-like subordinate clause.

Do not confuse it with потому что, which means because:

• Я знаю, почему ты опоздал. = I know why you were late.
• Ты опоздал, потому что был дождь. = You were late because it was raining.

Опоздал is the masculine singular past tense form of опоздать, meaning to be late or to arrive late.

Russian past tense agrees with gender and number:

• опоздал = masculine
• опоздала = feminine
• опоздало = neuter
• опоздали = plural

So:

• Теперь мне ясно, почему ты опоздал. = said to a male
• Теперь мне ясно, почему ты опоздала. = said to a female
• Теперь мне ясно, почему вы опоздали. = said to several people, or politely to one person

Yes, often it can.

Russian frequently drops subject pronouns when the meaning is clear from the verb form. So both are possible:

• Теперь мне ясно, почему ты опоздал.
• Теперь мне ясно, почему опоздал.

The version with ты is a bit more explicit and can add emphasis or clarity. In conversation, omitting it is very common if the context already shows who is being spoken to.

Yes. Russian word order is flexible, though some orders are more neutral than others.

The most neutral version is:

• Теперь мне ясно, почему ты опоздал.

Possible alternatives include:

• Мне теперь ясно, почему ты опоздал.
• Почему ты опоздал, теперь мне ясно.

These are all understandable, but they may shift emphasis slightly.

A rough idea:

• Теперь first = emphasizes now
• Мне earlier = emphasizes to me
• the clause first = emphasizes why you were late

For learners, the original order is the best one to use.

Yes, absolutely.

• Теперь мне ясно, почему ты опоздал.
• Теперь я понимаю, почему ты опоздал.

Both are natural and mean nearly the same thing.

The difference is small:

• мне ясно sounds a bit more like it is clear to me
• я понимаю sounds more direct: I understand

Another common option is:

• Теперь мне понятно, почему ты опоздал.

All three are natural.

They are similar, but not always exactly the same.

• опоздал = was late, failed to arrive on time
• пришёл поздно = came late / arrived late

If you are late for a meeting, class, or appointment, опоздал is often the most natural word.

For example:

• Ты опоздал на урок. = You were late for class.

Пришёл поздно can sometimes sound more descriptive and less tied to a scheduled time:

• Он пришёл поздно домой. = He came home late.

So in your sentence, опоздал is a very natural choice if the idea is that the person did not arrive when expected.

A very literal breakdown is:

• Теперь = now
• мне = to me
• ясно = clear
• почему = why
• ты = you
• опоздал = were late / arrived late

So literally:

Now to me [it is] clear why you were late.

This is a good example of how Russian often uses an impersonal structure where English prefers it is clear to me or I understand.