Learn Hardcore Indonesian: Orang dewasa yang melanggar aturan lalu lintas bisa mendapat denda besar dan harus belajar lebih hati-hati. - An adult who breaks traffic rules can receive a big fine and must learn to be more careful.

In English we use who/that/which; in Indonesian we usually use yang for all of these.
You cannot just say Orang dewasa melanggar aturan lalu lintas… if you want to specifically say adults who break traffic rules as a group being talked about; you need yang to connect the noun (orang dewasa) with the describing clause (melanggar aturan lalu lintas).

Is orang dewasa singular or plural? Why is there no word for a/an or the?

Orang dewasa is grammatically number-neutral. It can mean:

an adult
the adult
adults
the adults

In this sentence, the English translation is usually plural (Adults who break traffic rules…) because the idea is general: it’s talking about adults in general, not one specific person.

Indonesian normally does not mark singular/plural on nouns and doesn’t use articles like a/an or the. Context tells you whether it’s singular or plural.

If you really want to emphasize plural, you can say:

orang-orang dewasa – clearly plural (adults), but this sounds a bit heavy here and is not necessary.

Why is it orang dewasa, not dewasa orang?

In Indonesian, adjectives usually come after the noun they describe.

orang dewasa – adult person (literally: person adult)
denda besar – big fine (literally: fine big)

So:

orang = person
dewasa = adult / mature (adjective)

Together, orang dewasa literally means adult person, which we translate as adult or adults in English.
Putting dewasa before orang (dewasa orang) would be ungrammatical.

What is the difference between melanggar and the base word langgar?

The base word is langgar, meaning to violate / to break (a rule).

The prefix me- turns it into an active verb:

melanggar – to break/violate (actively do the action)

In standard Indonesian, you normally use the me- form when the subject actively does the verb:

Dia melanggar aturan. – He/She breaks/broke the rules.
Aturan dilanggar dia. – The rules are broken by him/her. (passive, using di-)

So melanggar is just the regular active-voice form built from langgar.

What exactly does aturan lalu lintas mean? What is lalu lintas?

Breakdown:

aturan = rule(s), regulation(s)
lalu lintas = traffic

Literally, lalu lintas combines:

lalu (to pass, to go past)
lintas (to cross, to pass through)

Together, lalu lintas becomes the fixed phrase traffic.

So:

aturan lalu lintas = traffic rules / traffic regulations

Why is it bisa mendapat denda besar and not something like will get a big fine? What does bisa add?

Bisa means can / be able to / may (possibility).

bisa mendapat denda besar = can get a big fine / may receive a big fine

This expresses possibility or potential consequence, not certainty.
If you said:

Orang dewasa yang melanggar aturan lalu lintas akan mendapat denda besar.

Using akan makes it sound like will definitely get a big fine (more certain, more like a rule that is always enforced).

Using bisa is more like:

They can get a big fine (this is one possible consequence).

What is the difference between mendapat and mendapatkan in this kind of sentence?

Both come from dapat (to get/obtain):

mendapat denda besar
mendapatkan denda besar

Here, both are acceptable and very similar in meaning: to get / to receive a big fine.

Nuance:

mendapat is slightly shorter and more common in everyday speech.
mendapatkan can feel a bit more formal or emphasize the act of obtaining, but in many cases they’re interchangeable.

Another common everyday alternative is:

bisa kena denda besar – can get hit with a big fine
bisa didenda – can be fined

Why is there no subject before harus belajar? Should there be a mereka?

The full idea in English is:

Adults who break traffic rules can get a big fine and (they) must learn to be more careful.

In Indonesian, if the subject stays the same, we often don’t repeat it after dan:

Orang dewasa yang melanggar aturan lalu lintas
bisa mendapat denda besar
dan harus belajar lebih hati-hati.

The understood subject (from context) is still orang dewasa yang melanggar aturan lalu lintas.

You could add mereka:

… bisa mendapat denda besar dan mereka harus belajar lebih hati-hati.

This is grammatically fine, but not necessary. The shorter version is more natural.

What does lebih do in belajar lebih hati-hati?

Lebih means more in comparisons or to show increased degree:

hati-hati – careful
lebih hati-hati – more careful

So belajar lebih hati-hati = learn to be more careful / learn to act more carefully.

Without lebih, belajar hati-hati would sound odd or incomplete here; lebih makes it clear they should increase their level of carefulness compared to before.

What does hati-hati literally mean, and why is it repeated?

Literally:

hati = liver/heart (in many Indonesian expressions it’s like “heart” in English idioms)
hati-hati (reduplicated) = careful, cautious; to be careful

Reduplication is very common in Indonesian and often adds nuances like:

repeated action
intensity
a change of word class (e.g., from noun to adjective/adverb-like)

Hati-hati is used:

as a description: Dia sangat hati-hati. – He/She is very careful.
as an imperative: Hati-hati di jalan! – Be careful on the road!

In belajar lebih hati-hati, it functions like an adjective/adverb: to be more careful / to act more carefully.

Why is there no word for to before belajar or before lebih hati-hati, like to learn to be more careful?

Indonesian doesn’t need a separate word like English to (infinitive marker) in this structure.

harus belajar lebih hati-hati
literally: must learn more careful

It is understood as:

must learn to be more careful / must learn to act more carefully

Indonesian simply puts the verbs/phrases one after another:

Dia mulai belajar menyetir. – He/She started to learn to drive.
Kita perlu belajar berbicara lebih pelan. – We need to learn to speak more slowly.

So you don’t say belajar untuk menjadi lebih hati-hati here unless you want a more formal or heavier style. The simple belajar lebih hati-hati is natural and clear.

Why is there no tense marker like will or must have? How do we know when this happens?

Indonesian verbs usually do not change for tense (past/present/future). Time is shown by:

context
time words (e.g., kemarin yesterday, besok tomorrow)
sometimes auxiliary words like sudah (already), akan (will)

Here:

bisa mendapat = can get / may get (in general)
harus belajar = must learn (in general)

The sentence expresses a general rule or consequence, not a specific time. In English, we naturally translate it as:

Adults who break traffic rules can get a big fine and must learn to be more careful.

If you needed to be specific about time, you’d add time words, for example:

Mulai tahun depan, orang dewasa yang melanggar aturan lalu lintas bisa mendapat denda besar…
From next year, adults who break traffic rules can get a big fine…