Learn Hardcore Hungarian: A barátom gyorsan megy a színházba, mert késik. - My friend is going to the theatre quickly because he is late.

In Hungarian, possessed nouns (like barátom – my friend) almost always take a definite article (a / az) when they are normal subjects or objects.

A barátom gyorsan megy… = My friend goes quickly…
Leaving out the article (Barátom gyorsan megy…) is possible but sounds more poetic, emphatic, or unusual in everyday speech.

So in neutral, everyday Hungarian, A barátom is the standard way to say my friend as a subject here.

What exactly does barátom mean, and how is it formed?

Barátom is made of:

barát = friend
-om = my (1st person singular possessive suffix)

So barátom literally = friend-my → my friend.

Hungarian usually uses possessive suffixes instead of separate words like my, your, etc.

Why is it gyorsan and not gyors before megy?

Gyors is an adjective (fast, quick) that describes a noun:

gyors autó = a fast car

Gyorsan is the adverb form (quickly, fast) that describes a verb:

gyorsan megy = (he/she) goes quickly

In Hungarian, adverbs of manner are often formed by adding -an / -en to the adjective:

lassú → lassan = slow → slowly
gyors → gyorsan = fast → quickly

Why is it megy and not something like megyen or another verb?

Megy is the 3rd person singular present tense of menni (to go):

én megyek – I go
te mész – you go
ő megy – he/she goes

The older form megyen exists in historical or very old-fashioned style, but in modern standard Hungarian you just say megy.

Also, Hungarian doesn’t have a separate “is going” tense. Megy can mean:

He/she goes (habitual)
He/she is going (right now)

Context tells you which is meant.

Why is it a színházba and not a színházban?

The endings show different directions/locations:

-ba / -be = into something (direction, to/into)
- a színházba megy = (he/she) goes to the theatre (into the building)
-ban / -ben = inside something (location, in)
- a színházban van = (he/she) is in the theatre

In the sentence, your friend is moving towards/into the theatre, so színházba (illative case: into) is correct.

Why is there a before színházba? In English, we say to the theatre, but sometimes also just to theatre.

Hungarian almost always uses a definite article (a / az) with specific, known places:

megyek a boltba = I’m going to the shop
megy a színházba = (he/she) is going to the theatre

Leaving out the article (megy színházba) is possible in special, fixed expressions or styles, but the neutral, everyday form is megy a színházba.

Why is the adverb gyorsan before megy? Could I say A barátom megy gyorsan a színházba?

The basic neutral order is:

A barátom gyorsan megy a színházba…
Subject – adverb – verb – place

You can say A barátom a színházba megy gyorsan, A barátom megy gyorsan a színházba, etc., but word order in Hungarian changes the focus/emphasis:

A barátom gyorsan megy a színházba…
Neutral: he’s going there quickly.
A barátom a színházba megy gyorsan…
Emphasis more on to the theatre (not somewhere else) as the place he is quickly going to.

So your original sentence is the most neutral and natural version.

Who is late in this sentence? My friend or the theatre?

It is your friend who is late.

A barátom gyorsan megy a színházba, mert késik.
= My friend is going quickly to the theatre because he/she is late.

The subject a barátom carries over to the second verb késik. Hungarian doesn’t repeat the pronoun ő here, because it’s clear the same person is being talked about.

Why is there no ő before késik? Why not … mert ő késik?

Hungarian usually drops subject pronouns when they are clear from context or verb endings.

(Ő) késik. = He/She is late.
A barátom gyorsan megy a színházba, mert (ő) késik.

Adding ő is possible, but in this sentence it would add extra emphasis:

… mert ő késik.
= because HE/SHE is late (as opposed to someone else).

The neutral, un-emphasized form is without ő.

What does késik literally mean? Is it a verb or an adjective?

Késik is a verb, not an adjective.

Infinitive: késni = to be late
késik = (he/she) is late / arrives late

So:

Ő késik. = He/She is late.
A busz késik. = The bus is late.

There is also the adjective késő (late), but you don’t say ő késő to mean “he is late” in the sense of “arrives late”. You use the verb késik.

Could we say mert elkésik instead of mert késik? What’s the difference?

Yes, you can, but there is a nuance:

mert késik
= because he/she is late (generic: is running late, is in a state of lateness)
mert elkésik
= because he/she will end up being late / will be late in the end
(el- adds a sense of completion/result)

In your sentence, mert késik is perfectly natural and means he/she is already late (or currently running late), and that’s why they’re hurrying.

Why is there a comma before mert?

Mert means because and introduces a subordinate clause explaining the reason.

Hungarian punctuation normally separates such a clause with a comma:

A barátom gyorsan megy a színházba, mert késik.
Elmegyek, mert fáradt vagyok. = I’m going away because I’m tired.

So the comma before mert is standard and correct.