Holnap is kötelező bemenni a tanfolyamra, ezért a fiú nem maradhat otthon.

Here is means also / too / as well.

So Holnap is means tomorrow too or tomorrow as well. The idea is that going to the course is required not only at some other time, but tomorrow as well.

A useful pattern is:

• ma is = today too
• holnap is = tomorrow too
• ő is = he/she too

Hungarian is usually comes directly after the word it belongs to.

Bemenni means to go in / to go inside, while menni just means to go.

In this sentence, bemenni a tanfolyamra means going in to attend the course. Hungarian often uses prefixed verbs like this to add direction or nuance:

• menni = to go
• bemennni = to go in
• elmenni = to go away
• felmenni = to go up

Here be- gives the sense of entering or going in.

This is an impersonal structure meaning it is obligatory to go in or going in is compulsory.

• kötelező = compulsory / obligatory
• bemenni = to go in

So literally it is something like:

• Holnap is kötelező bemenni a tanfolyamra
  = Tomorrow too, it is compulsory to go to the course

Hungarian often uses this kind of structure with an adjective plus infinitive:

• szabad bemenni = it is allowed to go in
• tilos dohányozni = smoking is forbidden
• nehéz megérteni = it is hard to understand

Because the first clause is phrased generally and impersonally.

Kötelező bemenni does not explicitly say who has to go in. It just states the rule or obligation in general: it is compulsory to go in.

Then the second clause makes the consequence specific:

• ezért a fiú nem maradhat otthon
  = therefore the boy cannot stay at home

So the logic is:

1. Going to the course tomorrow is compulsory.
2. Therefore, the boy cannot stay at home.

Because -ra/-re is the suffix used here for movement to something.

• tanfolyam = course
• tanfolyamra = to the course

This fits with a motion verb like bemenni.

Compare:

• tanfolyamra megy = he goes to the course
• tanfolyamon van = he is at the course / on the course

So:

• -ra/-re often marks direction toward a place/event
• -on/-en/-ön often marks location at a place

Also, the form is -ra rather than -re because tanfolyam has back vowels, so this follows vowel harmony.

Ezért means therefore, for this reason, or that is why.

It connects the two clauses:

• It is compulsory to go to the course tomorrow.
• Therefore, the boy cannot stay at home.

It comes from:

• ez = this
• -ért = for

So originally it is like for this, but in normal use it often functions as therefore.

Because maradhat contains the suffix -hat/-het, which often expresses can / may / be allowed to.

• marad = stays / remains
• maradhat = can stay / may stay / is allowed to stay

With negation:

• nem maradhat = cannot stay / may not stay / is not allowed to stay

In this sentence, the meaning is that because attendance is compulsory, the boy is not allowed to stay home.

The form is -hat rather than -het because marad has back vowels, so again this follows vowel harmony.

Because in Hungarian, nem normally comes directly before the finite verb when it negates it.

So:

• maradhat = can stay
• nem maradhat = cannot stay

This is the standard position for negation.

Compare:

• megy = he goes

• nem megy = he does not go

• jöhet = he may come
• nem jöhet = he may not come

Because otthon means at home, while haza means homeward / to home.

Here the sentence is about staying in a place, so the location form is needed:

• otthon marad = stays at home

Compare:

• hazamegy = goes home
• otthon van = is at home
• otthon marad = stays at home

So nem maradhat otthon correctly means he cannot stay at home.

A is the definite article, meaning the.

So:

• a fiú = the boy
• egy fiú = a boy

Hungarian has both definite and indefinite articles, similar to English:

• a / az = the
• egy = a / one

Here a fiú refers to a specific boy already known from the context.

Not completely. Hungarian word order is flexible, but different orders change the emphasis.

This version is natural because:

• Holnap is puts the time expression first as the topic
• ezért clearly introduces the consequence
• nem stands right before the verb it negates

You could rearrange parts of the sentence, but the emphasis would shift. For example, moving holnap is later might make the sentence sound less natural or put attention elsewhere.

So the sentence is not mechanically fixed, but the given order is a very normal and clear one.