Nem tudom pontosan, mikor érkezik a vonat, de valószínűleg délután.

Yes. Nem tudom literally means I don't know.

• nem = not
• tudom = I know it

More exactly, tudom is the 1st person singular definite form of tudni (to know). Hungarian often uses a definite verb form when the thing known is specific, even if it is expressed by a clause like mikor érkezik a vonat (when the train arrives / when the train is arriving).

So the first part means: Nem tudom pontosan = I don't know exactly

This is a very common learner question. Hungarian verbs can have two conjugations:

• indefinite conjugation
• definite conjugation

Here:

• tudok = I know / I can in a more general or indefinite sense
• tudom = I know it, where the object is definite or specific

In this sentence, what is being known is a specific piece of information: mikor érkezik a vonat (when the train arrives). Because of that, Hungarian uses the definite form tudom.

So:

• Nem tudok would not fit here.
• Nem tudom, mikor érkezik a vonat is the natural form.

Pontosan means exactly or precisely.

So:

• Nem tudom = I don't know
• Nem tudom pontosan = I don't know exactly

It softens the statement a little. The speaker is not saying they know nothing at all, only that they do not know the exact time.

Hungarian word order is flexible, but it is not random. It often reflects focus, emphasis, and information structure more than fixed grammatical position.

In this sentence, the order is natural and neutral:

• Nem tudom pontosan = the main statement
• mikor érkezik a vonat = the embedded clause explaining what is not known

You may see variations in other contexts, but this version is the standard, natural way to say it.

For example:

• Nem tudom pontosan, mikor érkezik a vonat.
  = neutral, natural

If you move things around, you may create emphasis or sound less natural unless the context supports it.

Mikor means when.

Here it introduces an indirect question:

• mikor érkezik a vonat = when the train arrives / when the train is arriving

This is not a direct question like:

• Mikor érkezik a vonat? = When does the train arrive?

Instead, it is embedded inside a larger sentence:

• Nem tudom, mikor érkezik a vonat.
  = I don't know when the train arrives.

So mikor works much like English when in I don't know when...

Érkezik is the 3rd person singular present tense form of érkezik (to arrive).

• érkezni = the infinitive, to arrive
• érkezik = he/she/it arrives

Since the subject is a vonat (the train), Hungarian uses the 3rd person singular form:

• a vonat érkezik = the train arrives

As for megérkezik: that verb can also mean arrives, often with a sense of finishing the arrival / getting there. In many contexts both are possible, but érkezik is very common when talking about scheduled arrival times, such as trains.

So:

• Mikor érkezik a vonat? = a very natural way to ask When does the train arrive?

Yes. A vonat means the train.

• a = the (used before consonant sounds)
• az = the (used before vowel sounds)
• vonat = train

In this sentence, the speaker is talking about a particular train relevant to the conversation, so Hungarian uses the definite article, just like English often would.

Examples:

• a vonat = the train
• egy vonat = a train

Here, a vonat is the natural choice.

Nothing is missing in Hungarian terms. Hungarian often leaves out words that are easy to understand from context.

The full idea is something like:

• ...de valószínűleg délután érkezik.
• ...but probably it arrives in the afternoon.

However, after the first clause already says érkezik a vonat, Hungarian can omit the repeated verb and subject because they are obvious.

So:

• de valószínűleg délután
  literally feels like: but probably in the afternoon

In natural English, we usually need to say:

• but probably in the afternoon or
• but it's probably in the afternoon depending on the intended meaning

This kind of ellipsis is very normal in Hungarian.

Valószínűleg means probably.

It comes from valószínű (probable / likely), plus the ending -leg, which forms an adverb.

So:

• valószínű = probable / likely
• valószínűleg = probably

In the sentence:

• de valószínűleg délután
  = but probably in the afternoon

It shows that the speaker is not certain, only making a likely guess.

This is a great point for English speakers. Hungarian often uses words for times of day without an extra suffix when they function adverbially.

• délután = afternoon / in the afternoon
• reggel = in the morning
• este = in the evening
• éjjel = at night

So in this sentence:

• valószínűleg délután = probably in the afternoon

No extra ending is needed.

Hungarian does sometimes use case endings with time expressions in other situations, but many common time words can stand alone as adverbs.

Yes. These forms express different time relationships.

• délután = in the afternoon
• délutánra = by the afternoon / for the afternoon
• délutánig = until the afternoon

Examples:

• Valószínűleg délután érkezik.
  = It will probably arrive in the afternoon.
• Délutánra megérkezik.
  = It will arrive by the afternoon.
• Délutánig várunk.
  = We are waiting until the afternoon.

So the basic form délután is correct here because it simply states the approximate time of arrival.

Yes. In English, the best translation depends on context.

Hungarian present tense érkezik can correspond to:

• arrives
• is arriving
• sometimes even a scheduled future sense like will arrive

Because this sentence is about a train's arrival time, English often uses:

• I don't know exactly when the train arrives, but probably in the afternoon. or
• I don't know exactly when the train will arrive, but probably in the afternoon.

Both can be appropriate depending on the situation.

Yes, it sounds natural.

• Nem tudom pontosan, mikor érkezik a vonat, de valószínűleg délután.

It is clear, idiomatic, and normal in conversation. A speaker might also say a slightly fuller version:

• Nem tudom pontosan, mikor érkezik a vonat, de valószínűleg délután érkezik.

That version repeats the verb, but the shorter one is more natural because repetition is unnecessary.

A rough English-friendly guide:

• Nem tudom ≈ nem TOO-dom
• pontosan ≈ PON-toh-shahn
• mikor ≈ MEE-kor
• érkezik ≈ AIR-keh-zik
• a vonat ≈ aw VO-nawt
• valószínűleg ≈ vaw-LOH-see-new-leg
• délután ≈ DAY-loo-taan

A few important notes:

• s in Hungarian sounds like English sh
• sz sounds like English s
• a is usually closer to a short open o sound
• stress is almost always on the first syllable

So:

• VALÓSZÍNŰLEG has stress on va
• DÉLUTÁN has stress on dél

Not in this sentence.

• mikor = when in a question or indirect question
• amikor = when in a relative sense, more like at the time when

Here, the speaker does not know the time, so it is an indirect question:

• Nem tudom, mikor érkezik a vonat.
  = I don't know when the train arrives.

Using amikor here would sound wrong.

Compare:

• Mikor érkezik a vonat? = When does the train arrive?
• Amikor a vonat érkezik, kimegyek az állomásra.
  = When the train arrives, I go out to the station. / At the time when the train arrives...

So mikor is the correct choice in your sentence.