Learn Hardcore Indonesian: Kadang-kadang dia merasa cemburu ketika temannya lebih cepat berhasil. - Sometimes he/she feels jealous when his/her friend succeeds faster.

In standard/formal writing, you should prefer kadang-kadang with a hyphen. Writing only one kadang is also correct but slightly more informal in feeling.

Can I put kadang-kadang in other positions in the sentence?

Yes. In Indonesian, time adverbs like kadang-kadang are flexible. All of these are natural:

Kadang-kadang dia merasa cemburu ketika temannya lebih cepat berhasil.
Dia kadang-kadang merasa cemburu ketika temannya lebih cepat berhasil.
Dia merasa cemburu kadang-kadang ketika temannya lebih cepat berhasil. (less common, but still understandable)

Most natural are the first two, with kadang-kadang at the beginning or right after dia.

Why do we need merasa before cemburu? Can I just say dia cemburu?

You can say both:

Dia merasa cemburu = literally “he/she feels jealous”
Dia cemburu = “he/she is jealous”

merasa means to feel, so merasa cemburu emphasizes the internal feeling.
Dia cemburu is shorter and very common in everyday speech.

In many contexts, there is almost no difference in meaning; merasa can often be omitted with emotion adjectives (cemburu, sedih, marah, senang, etc.) unless you want to highlight the act of feeling.

What is the difference between cemburu and iri?

Both relate to jealousy/envy, but there is a nuance:

cemburu: often used for romantic or possessive jealousy.
- Dia cemburu karena pacarnya dekat dengan orang lain.
iri: envy/jealousy about someone else’s advantages, success, or possessions.
- Dia iri karena temannya lebih kaya.

In this sentence about a friend succeeding faster, iri would also be possible:

Kadang-kadang dia merasa iri ketika temannya lebih cepat berhasil.

Many speakers use cemburu and iri quite loosely, so you will hear some overlap.

Does dia mean “he” or “she”? How do you know the gender?

Dia can mean he or she. Indonesian third-person pronouns do not mark gender.

You find out the gender from:

context,
previous sentences,
names or other information (e.g. Dia adalah kakak perempuan saya – "She is my older sister").

Without context, dia is simply he/she.

What does the -nya in temannya mean? Is it “his friend” or “her friend”?

The suffix -nya here works as a possessive and can mean:

his friend
her friend

Since dia is gender-neutral, temannya is also gender-neutral: “his/her friend”.

Depending on context, -nya can also mean the (a definite thing), but in this sentence, it naturally reads as “his/her friend”.

Can temannya also mean “their friends”? How do I say “friends”?

temannya by itself is singular in meaning: his/her friend or the friend.

To clearly show plurals:

teman-temannya = his/her friends / their friends
- reduplication (teman-teman) marks plural
- -nya still marks possession

Examples:

Dia sedih karena teman-temannya pindah.
= He/She is sad because his/her friends moved away.

If you only say temannya, you probably mean just one friend.

How does lebih cepat berhasil work grammatically? Which word is being compared?

Breakdown:

lebih = more
cepat = fast/quick(ly)
berhasil = to succeed / be successful

So lebih cepat berhasil literally means to succeed more quickly / to succeed faster.

Grammatically:

lebih makes a comparative: “more + adjective/adverb” (lebih cepat = faster / more quickly)
cepat describes how the person succeeds.
berhasil is the verb “to succeed”.

The implied comparison is usually “compared to him/her” or “compared to others”, even if it isn’t stated.

Could I say berhasil lebih cepat instead of lebih cepat berhasil?

Yes, you can. Both are acceptable:

ketika temannya lebih cepat berhasil
ketika temannya berhasil lebih cepat

They mean essentially the same: when his/her friend succeeds faster.

Nuance:

lebih cepat berhasil slightly emphasizes the speed before the success.
berhasil lebih cepat slightly emphasizes the success first, then adds the detail that it is faster.

In everyday conversation, both sound natural.

What is the difference between ketika, saat, waktu, and kalau here?

All of these can introduce a time clause, but with different nuances:

ketika – neutral “when (at the time that)”.
saat – very similar to ketika, slightly more formal/literary in many contexts.
waktu – literally “time”; used like when in spoken Indonesian.
kalau – normally means “if”, but often used in speech where English uses “when”.

In this sentence, you can say:

… ketika temannya lebih cepat berhasil. (neutral)
… saat temannya lebih cepat berhasil. (also neutral/natural)
… waktu temannya lebih cepat berhasil. (very common, conversational)

Using kalau here would sound more like “if his/her friend succeeds faster” (conditional), which slightly changes the meaning.

How do you make this sentence past tense or future tense in Indonesian?

Indonesian does not change the verb form for tense. You add time words if you need to be clear.

Base sentence (general/habitual):

Kadang-kadang dia merasa cemburu ketika temannya lebih cepat berhasil.
= Sometimes he/she feels jealous when his/her friend succeeds faster.

Past (add a past time marker):

Dulu kadang-kadang dia merasa cemburu ketika temannya lebih cepat berhasil.
= In the past, sometimes he/she felt jealous…

Future (add a future time marker and often akan):

Nanti kadang-kadang dia akan merasa cemburu ketika temannya lebih cepat berhasil.
= In the future, sometimes he/she will feel jealous…

So tense is shown by words like dulu, tadi, kemarin, besok, nanti, akan, etc., not by changing merasa or berhasil.