Learn Hardcore Indonesian: Dia mencoba tetap optimis, sementara sahabatnya yang pesimis menyiapkan rencana cadangan yang sangat detail. - She/He tries to stay optimistic, while her/his pessimistic best friend prepares a very detailed backup plan.

Indonesian third‑person pronouns (dia, ia, beliau) do not mark gender. You only know if it’s male or female from context, for example if earlier the text mentioned seorang pria (a man) or seorang perempuan (a woman). The capital D here is only because it starts the sentence, not because of respect or anything special.

Why is it mencoba tetap optimis and not mencoba untuk tetap optimis? Is untuk wrong here?

Both mencoba tetap optimis and mencoba untuk tetap optimis are grammatically correct.

mencoba tetap optimis = try to stay optimistic (more direct, very natural in speech)
mencoba untuk tetap optimis = also try to stay optimistic, a bit more formal or careful style

In many verb + verb patterns, untuk is optional after mencoba. Leaving untuk out does not change the meaning here.

What exactly does tetap mean in tetap optimis? Could we use terus instead?

tetap means to remain / to stay (in the same state). tetap optimis = remain optimistic, keep being optimistic, stay optimistic (without changing).

terus means continuously, go on, keep (doing something) and often focuses on time or repetition.

Dia mencoba tetap optimis – he/she tries not to lose optimism, not to change his/her attitude.
Dia mencoba terus optimis – he/she tries to be optimistic all the time / continuously.

In this sentence, tetap is more natural because the idea is “don’t stop being optimistic” rather than “be optimistic non‑stop”.

What is the function of sementara in this sentence? Is it closer to while or whereas in English? Could I use sedangkan instead?

Here sementara works like while or whereas, showing contrast between two people:

Dia mencoba tetap optimis, sementara sahabatnya…
→ He/She tries to stay optimistic, while/whereas his/her friend…

So it is both temporal (at the same time) and contrastive (but in contrast).

You can usually replace it with sedangkan in this type of contrast sentence:

Dia mencoba tetap optimis, sedangkan sahabatnya yang pesimis…

That would also be natural. sementara is a bit more neutral (it can mean “while/whereas” or “meanwhile”), while sedangkan is used more specifically for whereas/on the other hand contrasts.

What is the difference between sahabat and teman? Why is sahabatnya used here?

Both mean friend, but the nuance is different:

teman – general friend / companion / acquaintance. Very common and neutral.
sahabat – close friend, best friend, someone with a deeper, long‑term bond.

So sahabatnya suggests his/her close friend / best friend, not just any casual friend. The choice of sahabat helps show that the contrast is between two people who are close to each other, not strangers.

How does -nya work in sahabatnya? Does it always mean his/her?

In sahabatnya, -nya marks third‑person possession: his/her friend. So sahabatnya = his friend / her friend / their friend (singular).

In general, -nya has several functions in Indonesian:

Possessive: bukunya (his/her book), rumahnya (his/her house).
Definite marker: rumahnya besar can also mean the house is big, where -nya makes rumah specific.
Third‑person object pronoun: Saya melihatnya (I saw him/her/it).

In this sentence it’s clearly possessive: sahabatnya = his/her close friend.

What is yang doing in the phrase sahabatnya yang pesimis? Could we just say sahabatnya pesimis?

yang here introduces a descriptive clause/phrase that modifies sahabatnya:

sahabatnya yang pesimis = his/her friend who is pessimistic / his/her pessimistic friend.

If you remove yang, the structure changes:

sahabatnya pesimis is a full sentence: His/her friend is pessimistic.
(subject = sahabatnya, predicate = pesimis).

So:

With yang → one long noun phrase: sahabatnya yang pesimis (used as the subject of menyiapkan).
Without yang → you get two separate clauses, which doesn’t fit the original sentence structure.

That’s why yang is needed here.

Why does the adjective pesimis come after sahabatnya and not before, like in English (pessimistic friend)?

In Indonesian, adjectives normally come after the noun they describe:

rumah besar – big house
buku baru – new book
teman lama – old friend

So sahabatnya yang pesimis literally has the structure friend (of his/hers) who pessimistic, which in English becomes his/her pessimistic friend.

Putting the adjective before the noun (pesimis sahabatnya) is not normal Indonesian grammar in this context.

What does rencana cadangan literally mean, and how is it used? Is it the same as plan B?

Literally:

rencana = plan
cadangan = reserve, backup, spare

So rencana cadangan = backup plan / reserve plan.

In usage, it’s very close to English plan B:

Kita perlu rencana cadangan kalau ini gagal.
→ We need a backup plan / plan B if this fails.

Other close expressions: rencana lain (another plan), rencana alternatif (alternative plan), but rencana cadangan is the standard phrase for a backup.

In rencana cadangan yang sangat detail, why is there yang before sangat detail? Could we omit it?

Here yang again links the noun phrase to a description:

rencana cadangan yang sangat detail
= a backup plan that is very detailed / a very detailed backup plan.

This yang turns sangat detail (very detailed) into a modifier of rencana cadangan.

If you remove yang:

rencana cadangan sangat detail is more like a full clause: the backup plan is very detailed.
It can work in some contexts, but inside this longer sentence, the yang version is smoother and clearly makes the whole chunk one noun phrase that functions as the object of menyiapkan.

So you can sometimes omit yang, but here yang is more natural and clearly grammatical.

Is detail in sangat detail a noun or an adjective in Indonesian? Is there any difference between detail and rinci?

In sangat detail, detail is used as an adjective meaning detailed, not as a noun meaning detail (piece of information).

sangat detail = very detailed
penjelasan detail = a detailed explanation

detail is a loanword (from Dutch/English) and is very common, especially in modern and informal language.

rinci is a more native‑sounding word with a similar meaning (detailed), and secara rinci = in detail.

In this sentence, both would be fine:

rencana cadangan yang sangat detail
rencana cadangan yang sangat rinci

detail sounds a bit more casual/modern; rinci a bit more formal or “textbook‑like,” but both are natural.

What does menyiapkan mean exactly here, and how is it different from mempersiapkan?

Both menyiapkan and mempersiapkan come from the root siap (ready), and both mean to prepare.

menyiapkan rencana cadangan = to prepare a backup plan
mempersiapkan rencana cadangan = also to prepare a backup plan

Differences:

menyiapkan is a bit shorter and very common in everyday language.
mempersiapkan often sounds slightly more formal or emphasizes the process of preparation a little more, but in many contexts they are interchangeable.

In this sentence, menyiapkan is perfectly natural and probably the most typical choice.

Could the sentence start with Sementara sahabatnya … instead of Dia mencoba … sementara …? Would the meaning change?

Yes, you can flip the order:

Sementara sahabatnya yang pesimis menyiapkan rencana cadangan yang sangat detail, dia mencoba tetap optimis.

The basic meaning is the same. The difference is just what you emphasize first:

Original: starts with Dia mencoba tetap optimis → first focus on the optimistic person.
Reordered: starts with Sementara sahabatnya… → first focus on the pessimistic friend and the detailed backup plan.

Both orders are grammatically correct and natural in Indonesian.