A tanfolyamon kötelező időben érkezni.

The base word is tanfolyam, which means course, training course, or sometimes class.

The ending -on is a case ending called the superessive. It often means on, but in real usage it can also correspond to English at or in, depending on the noun.

So:

• tanfolyam = course
• tanfolyamon = on the course / in the course / at the course

With things like courses, lessons, meetings, and events, Hungarian often uses this ending where English would naturally say in or at.

Because Hungarian has two forms of the definite article:

• a before a consonant sound
• az before a vowel sound

Since tanfolyamon starts with t, a consonant, the correct form is a.

So:

• a tanfolyamon = on/in the course
• az iskolában = in the school

Kötelező means compulsory, mandatory, or required.

In this sentence, it works as the main predicate of the statement. The idea is:

• A tanfolyamon kötelező... = In this course, it is mandatory...

So the sentence is expressing a rule, not describing someone personally.

Hungarian often leaves out the verb to be in the present tense, especially in the third person singular when the predicate is a noun or adjective.

So instead of saying something like it is mandatory, Hungarian can simply say:

• kötelező = is mandatory

That is why there is no separate word here for is. The sentence is perfectly normal without it.

Because Hungarian commonly uses the pattern:

• kötelező + infinitive

This means it is mandatory to do something.

So:

• érkezni = to arrive
• kötelező érkezni = it is mandatory to arrive

This is very similar to English structure. The sentence is talking about the action in general, not about a specific person with a conjugated verb.

Időben means in time or on time.

It comes from:

• idő = time
• -ben = in

So literally it is something like in time. In the context of arriving, the most natural English meaning is on time.

So:

• időben érkezni = to arrive on time

It usually means not late, rather than exactly at the precise minute.

Not quite.

• időben = on time, in time, not late
• pontosan = exactly, precisely, punctually

So if you say:

• időben érkezni = arrive on time
• pontosan érkezni = arrive exactly/punctually

In many situations they are close, but időben is the more natural word for a rule about not being late.

The subject is left unstated because Hungarian often does that in general rules, notices, and instructions.

So the sentence really means something like:

• participants must arrive on time
• you have to arrive on time
• one must arrive on time

The exact subject is understood from context. Since the sentence is about the course, it obviously applies to the people attending it.

This is a natural neutral word order.

• A tanfolyamon sets the context: as for the course / in the course
• kötelező gives the main statement: it is mandatory
• időben érkezni tells you what is mandatory: to arrive on time

So the sentence flows as:

In the course, it is mandatory to arrive on time.

Hungarian word order is flexible, but changes often affect emphasis. This version sounds like a normal rule or regulation.

Yes, a very similar sentence would be:

• A tanfolyamon időben kell érkezni.

That also means You must arrive on time in the course / for the course.

The difference is mainly style:

• kötelező sounds a bit more formal and rule-based: it is mandatory
• kell is the more basic must / have to

So kötelező fits especially well in instructions, regulations, or official course requirements.