Learn Hardcore Hungarian: Pénteken vonattal megyek a tengerpartra.

The basic suffix for “on” a time expression is -n, but in many words a linking vowel appears between the stem and the suffix so that it’s pronounceable.

Base: péntek
Add suffix: péntek + -n
Pronunciation would be awkward as péntekn, so Hungarian inserts -e-:
pénteken

So -en here is really -e- (linking vowel) + -n (the actual suffix).

Why is pénteken capitalized? Are days of the week always capitalized in Hungarian?

Hungarian normally writes days of the week with a lowercase initial:

hétfő, kedd, szerda, csütörtök, péntek, szombat, vasárnap

In the sentence Pénteken vonattal megyek a tengerpartra., Pénteken is capitalized only because it is the first word of the sentence, not because it’s a day of the week.
If it appeared in the middle of a sentence, it would be pénteken.

Why is there no article (like “the”) before pénteken?

English often says on Friday, not on the Friday, and Hungarian behaves similarly with days of the week used as time expressions:

Pénteken megyek… = I’m going on Friday.
Not: A pénteken megyek… (that would usually be wrong or highly marked).

So days used as adverbials of time generally appear without articles in Hungarian.

What does vonattal literally mean, and why is it used for “by train”?

Vonat = train
The suffix -val / -vel expresses:

instrument (“with, by means of”)
or accompaniment (“together with”)

vonat + -val → vonattal (the v assimilates to t, doubling it)

So vonattal literally means “with a train / by means of a train”, which is how Hungarian says “by train” (as a means of transport).

Why is it vonattal and not vonaton?

Both are correct but mean different things:

vonattal = by train (means of transport, instrumental case)
vonaton = on the train (location, superessive case: “on” something)

In this sentence, we care about the means of going (how?), not the location, so vonattal is used.

Is the subject “I” expressed anywhere? Why is there no én?

Hungarian is a pro-drop language: the verb ending already shows who the subject is, so you can omit the pronoun if it’s obvious.

megyek = I go / I am going
- stem: megy-
- ending: -ek (1st person singular)

So (Én) vonattal megyek… is possible, but the én is usually left out unless you want to emphasize I (as opposed to someone else).

What tense is megyek? Why does it mean “I’m going” / “I will go” even though it looks present tense?

Megyek is present tense, indicative, 1st person singular.

Hungarian often uses the present tense + a time expression to talk about the future:

Holnap moziba megyek. = I’m going to the cinema tomorrow / I will go to the cinema tomorrow.
Pénteken vonattal megyek… = On Friday I’m going by train / I will go by train.

A specifically “future” form (menni fogok) exists but sounds more explicit or emphatic, and is not needed in neutral sentences like this.

Why is it a tengerpartra and not just tengerpartra?

A is the definite article (“the”). Here we mean the seaside/coast (some specific or understood coastline), not just “to a seaside” in a general, abstract sense.

a tengerpart = the seaside / the seashore
a tengerpartra = to the seaside (to the seashore)

In Hungarian, place nouns that are concrete and specific almost always take an article, even where English might sometimes omit the.

What is the base form of tengerpartra, and what does the -ra mean?

The base noun is tengerpart:

tenger = sea
part = shore, bank, coast
tengerpart = seaside / seashore (a compound word)

The ending -ra / -re is the sublative case: “onto / to (a surface or place)”.
Because tengerpart has back vowels (a), it takes the back-vowel variant:

tengerpart + -ra → tengerpartra = to the seaside

So a tengerpartra = “to the seaside”.

Why is it -ra and not -re in tengerpartra?

Hungarian suffixes follow vowel harmony:

Words with predominantly back vowels (a, á, o, ó, u, ú) take back-vowel suffixes (-ra).
Words with predominantly front vowels (e, é, i, í, ö, ő, ü, ű) take front-vowel suffixes (-re).

tengerpart contains the back vowel a, so it takes -ra:

tengerpartra = to the seaside
Compare with: kert → kertre (garden → onto the garden), with front vowel e.

Could the word order be Pénteken megyek vonattal a tengerpartra instead? Does that change the meaning?

Yes, that word order is also grammatically correct:

Pénteken vonattal megyek a tengerpartra.
Pénteken megyek vonattal a tengerpartra.

Both can mean the same thing in a neutral context: “On Friday I’m going to the seaside by train.”

Hungarian word order is relatively flexible and mainly used for information structure (what is emphasized, what is new information).
In isolation, without context, both versions sound natural and the difference in emphasis is minor.

Where is the verb placed in Hungarian? Does it always come in the middle like megyek does here?

Hungarian does not have a strict fixed position for the verb like English.
Instead, the word order is organized around a focus position, usually immediately before the verb, and the verb often follows that focused element.

Examples (all correct, with slight differences in emphasis):

Pénteken vonattal megyek a tengerpartra. (neutral, listing time and manner before the verb)
Pénteken a tengerpartra megyek vonattal. (focus on destination before the verb)
Vonattal megyek pénteken a tengerpartra. (highlighting “by train”)

So megyek tends to come after the element that’s in focus, but its exact position can vary.

How would you say “I go to the seaside on Fridays (regularly)” in Hungarian? Is pénteken still correct?

For a habitual action on Fridays, Hungarian usually uses the plural form of “Friday”:

Pénteken vonattal megyek a tengerpartra. = I’m going to the seaside by train on Friday (this particular Friday).
Péntekente vonattal járok a tengerpartra. = I go (regularly) to the seaside by train on Fridays.

-ente / -ánként is a suffix meaning “on X-s, every X”.
So péntekente ≈ “on Fridays / every Friday”.