Kadang saya iri melihat teman yang fasih berbicara, tetapi saya tetap berlatih.

Kadang means sometimes.

Yes, kadang and kadang-kadang are very close in meaning. In many contexts, they are interchangeable.

• Kadang = sometimes
• Kadang-kadang = sometimes, occasionally

The reduplicated form can sound a little more complete or a little more natural in some everyday speech, but kadang is perfectly normal.

So:

• Kadang saya iri... = Sometimes I feel jealous...
• Kadang-kadang saya iri... = same basic meaning

In Indonesian, you often do not need a verb like to be.

So:

• saya iri literally looks like I jealous
• but naturally it means I am jealous or I feel jealous

This is very common in Indonesian:

• Saya senang = I am happy
• Dia lelah = He/She is tired
• Mereka siap = They are ready

English requires am/is/are, but Indonesian usually does not.

Iri often covers both jealous and envious in everyday translation.

In this sentence, it is closer to envious:

• Kadang saya iri melihat teman yang fasih berbicara
  = Sometimes I feel envious when I see friends who speak fluently

But many learners will still translate it as jealous, and that is common.

So the important point is that iri expresses a feeling of wanting what someone else has or feeling bothered by their advantage.

Yes, more or less.

Melihat means to see. In this sentence, it explains when or in what situation the feeling happens:

• iri melihat teman... = jealous when seeing friends...
• more natural English: jealous when I see friends...

So Indonesian can use a verb phrase like this very directly:

• Saya senang mendengar itu = I’m happy to hear that
• Saya sedih melihatnya = I’m sad to see it/him/her
• Saya iri melihat teman... = I’m jealous seeing / when seeing friends...

It is natural Indonesian, even if the literal structure feels a little different from English.

Yes.

You could say:

• Saya iri pada teman yang fasih berbicara
• Saya iri kepada teman yang fasih berbicara

This means I’m jealous of friends who speak fluently.

The version in your sentence, iri melihat teman..., focuses a bit more on the feeling arising when seeing them.
The version with pada/kepada focuses more directly on who the jealousy is directed toward.

So both are possible, but they are not exactly identical in nuance.

Yang introduces a relative clause, similar to who, that, or which in English.

So:

• teman = friend / friends
• teman yang fasih berbicara = friend(s) who speak fluently

This is a very common pattern in Indonesian:

• orang yang baik = a person who is kind
• buku yang saya baca = the book that I read
• murid yang rajin belajar = students who study diligently

In your sentence, yang fasih berbicara describes teman.

Yes. Teman can mean friend or friends.

Indonesian nouns usually do not have to show singular vs. plural. Context tells you.

So:

• teman = friend / friends
• teman-teman = friends, explicitly plural

In this sentence, English might translate it as either:

• a friend who speaks fluently
• friends who speak fluently

Because the sentence is general, friends is often the most natural English translation.

Fasih means fluent, especially in speaking.

So:

• fasih berbicara = fluent in speaking
• natural English: speak fluently or be fluent when speaking

A few related examples:

• Dia fasih berbahasa Indonesia = He/She is fluent in Indonesian
• Dia fasih berbicara di depan umum = He/She speaks fluently in public

In your sentence, teman yang fasih berbicara means friends who can speak smoothly and confidently.

Both are possible, but berbicara is more standard and a bit more formal.

• bicara = talk, speak
• berbicara = to speak, to talk

In everyday conversation, people often use bicara.
In more careful or standard Indonesian, berbicara is very common.

So:

• fasih berbicara sounds standard and natural
• fasih bicara may be heard in speech, but fasih berbicara is safer for learners

The prefix ber- often forms an intransitive verb, often meaning something like do, have, engage in, or perform the action.

In your sentence:

• berbicara = to speak
• berlatih = to practice / to train

Other examples:

• berjalan = to walk
• berenang = to swim
• bekerja = to work

You do not need to translate ber- with one fixed English word. Its role is grammatical: it helps make a verb.

Tetap here means still, nevertheless, or continue to depending on how you translate it.

So:

• saya tetap berlatih = I keep practicing
• or I still practice
• or I continue practicing

In this sentence, tetap shows persistence despite the earlier feeling:

• Sometimes I feel jealous..., but I keep practicing.

So it carries the idea of not giving up.

Tetapi and tapi both mean but.

The difference is mainly style:

• tetapi = more formal or standard
• tapi = more casual and common in speech

In writing, especially careful writing, tetapi is often preferred.
In conversation, tapi is extremely common.

So your sentence sounds slightly more polished because it uses tetapi.

It is repeated because Indonesian often states the subject clearly in each clause, especially in standard writing.

So:

• Kadang saya iri melihat teman yang fasih berbicara, tetapi saya tetap berlatih.

This is clear and balanced:

• clause 1: saya iri
• clause 2: saya tetap berlatih

You might hear omission in casual speech if the subject is obvious, but repeating saya is very natural and correct here.

Yes.

Both are natural:

• Kadang saya iri melihat teman yang fasih berbicara...
• Saya kadang iri melihat teman yang fasih berbicara...

The difference is small:

• Kadang saya... puts a bit more focus on sometimes
• Saya kadang... starts more directly with I

Indonesian often allows this kind of flexibility, especially with time or frequency words like:

• kadang
• sering
• jarang
• selalu

So both versions are grammatical and natural.