Learn Hardcore Indonesian: Cairan di tabung kaca itu kelihatan menakutkan bagi adik saya. - The liquid in that glass tube looks scary to my younger sibling.

It doesn’t show singular or plural by itself (Indonesian usually doesn’t mark plural on nouns).
Context will tell you if it’s a liquid or liquids.
If you really need to stress plurality, you can say berbagai cairan (various liquids) or cairan-cairan (liquids, more formal/written style).

In this sentence, Cairan di tabung kaca itu… is naturally understood as The liquid in that glass tube….

Why is it di tabung kaca itu and not di dalam tabung kaca itu for “in the glass tube”?

Both are possible, but there’s a nuance:

di tabung kaca itu
Literally “at/on/in that glass tube”. Di is very flexible and often used where English uses in, on, or at. This is normal and not wrong.
di dalam tabung kaca itu
Adds dalam (inside), so it explicitly means inside that glass tube. It feels a bit more explicit or descriptive.

In everyday speech, Indonesians often just use di where English would use in, especially when the container relationship (like liquid ↔ tube) is obvious from context.

Why is it tabung kaca and not kaca tabung if the meaning is “glass tube”?

In Indonesian, the general order is:

Head noun + describing noun/adjective

So:

tabung kaca = tube (made of) glass → glass tube
meja kayu = wooden table
rumah batu = stone house

Putting kaca before tabung (like kaca tabung) would sound odd or change the meaning; it might be interpreted more like “glass of a tube” rather than glass tube.

So tabung kaca is the natural way to say glass tube.

What does itu do in tabung kaca itu? Is it like “the” or “that”?

Itu is a demonstrative that usually means that (as opposed to ini = this), but it also often works like a marker of known/definite things, a bit like the.

In this sentence:

tabung kaca itu = that glass tube / the glass tube (we’ve been talking about or can see)

So itu can indicate:

Physical distance: that thing over there
Or shared knowledge: the specific one we both know about

What does kelihatan mean, and how is it different from terlihat, tampak, or nampak?

Kelihatan means (to) look / appear / seem (visually).

Roughly:

kelihatan: common, neutral, everyday
- Cairan itu kelihatan menakutkan. = The liquid looks scary.
terlihat: similar meaning; sometimes feels slightly more formal or neutral than kelihatan, but in many contexts they’re interchangeable.
tampak / nampak: also mean to appear / to be visible, often a bit more formal or literary, but also used in speech.

For this sentence, you could also say:

Cairan di tabung kaca itu terlihat menakutkan bagi adik saya.
Cairan di tabung kaca itu tampak menakutkan bagi adik saya.

All are acceptable. Kelihatan is a very natural, conversational choice.

What’s the difference between takut, menakutkan, and menakuti?

All are related to fear, but they have different roles:

takut = afraid (adjective/verb)
- Adik saya takut. = My younger sibling is afraid.
menakutkan = frightening / scary (adjective or verb, “to frighten”)
- As an adjective:
  Film itu menakutkan. = That movie is scary.
- In your sentence:
  Cairan … kelihatan menakutkan = The liquid looks frightening.
menakuti = to frighten/scare someone (focus on the person being scared)
- Cairan itu menakuti adik saya. = The liquid scares my younger sibling.

So in kelihatan menakutkan, menakutkan functions like an adjective: scary.

What does bagi mean here, and could we use untuk or buat instead?

Bagi in bagi adik saya means for / to (in the sense of “in someone’s opinion / perspective”).

The structure X menakutkan bagi Y ≈ X is frightening to Y.

Alternatives:

untuk adik saya – also possible, a bit more general (“for my younger sibling”). It can still mean from their point of view, but bagi is slightly more “opinion/perspective”-oriented.
buat adik saya – very colloquial/informal version of untuk.

All of these can work in many real sentences, but here:

bagi adik saya feels slightly more formal and clearly expresses “from my younger sibling’s perspective.”

What exactly does adik mean? Is it “younger brother,” “younger sister,” or just “younger sibling”?

Adik means younger sibling, without specifying gender.

To be specific:

adik laki-laki = younger brother
adik perempuan = younger sister

In everyday conversation, context usually makes gender clear, or speakers just don’t care to specify.

So adik saya can mean:

my younger sibling (gender unspecified)
or my little brother / little sister, depending on context.

Why do we say adik saya instead of a separate word for “my,” like in English?

Indonesian typically marks possession by putting the possessor after the possessed noun.

Pattern:

[thing] + [possessor]

Examples:

adik saya = my younger sibling
buku saya = my book
rumah mereka = their house

You can also attach possessive suffixes in informal speech:

adikku, bukuku, rumahnya, etc.

In this sentence, adik saya is the neutral, clear way to say my younger sibling.

Could we say Adik saya takut pada cairan di tabung kaca itu instead? What’s the difference in meaning?

Yes, that’s grammatical and natural, but the focus shifts:

Cairan di tabung kaca itu kelihatan menakutkan bagi adik saya.
– Subject: the liquid
– Meaning focus: The liquid looks scary (from my sibling’s point of view).
– Emphasizes the appearance of the liquid.
Adik saya takut pada cairan di tabung kaca itu.
– Subject: my younger sibling
– Meaning focus: My younger sibling is afraid of the liquid in the glass tube.
– Emphasizes the emotion of the sibling.

Both describe a similar situation, but the original sentence highlights what the liquid looks like rather than the fact that the sibling is afraid.

How do we know if this sentence is present, past, or future? There’s no tense marking like in English.

Indonesian verbs generally don’t change form for tense. Time is shown by context or by adding time words:

Present (general):
Cairan di tabung kaca itu kelihatan menakutkan bagi adik saya.
→ The liquid looks scary (now / in general).
Past:
Tadi cairan di tabung kaca itu kelihatan menakutkan bagi adik saya.
(tadi = earlier/just now) → The liquid looked scary.
Future:
Nanti cairan di tabung kaca itu akan kelihatan menakutkan bagi adik saya.
(nanti / akan = later / will) → The liquid will look scary.

Without extra words, this sentence is most naturally understood as describing a present or general state.

Is this sentence formal, neutral, or casual? How might people say it in more casual Indonesian?

As written, it’s neutral and acceptable in most spoken and written contexts.

A more casual/colloquial version might look like:

Cairan di tabung kaca itu kelihatannya serem buat adik aku.
- kelihatannya = looks / seems (colloquial)
- serem = slangy for menakutkan
- buat = informal untuk
- adik aku = informal version of adik saya

The original sentence is good standard Indonesian—neither very formal nor very slangy.

Can we drop some parts, like bagi adik saya or itu? How does that change the meaning?

Yes:

Cairan di tabung kaca itu kelihatan menakutkan.
– Removes bagi adik saya.
– Meaning: The liquid in that glass tube looks scary.
– No mention of who finds it scary; it’s more general.
Cairan di tabung kaca kelihatan menakutkan bagi adik saya.
– Removes itu.
– Meaning: The liquid in the/a glass tube looks scary to my younger sibling.
– Feels less specific; could be any glass tube, not a particular one we both know.

The full sentence with itu and bagi adik saya is specific about which tube and whose perspective.