Ketika ombak mulai besar, jangkar kapal belum diangkat dan beberapa penumpang merasa mabuk laut.

Ketika introduces a time clause, like when in English. It tells you the situation in which the rest of the sentence happens.

So the pattern is:

Ketika + clause, main clause

In this sentence, Ketika ombak mulai besar sets the scene first, and then the main events follow.

You could also often use saat or waktu in similar contexts, though ketika can sound a bit more formal or narrative.

Both can work, but they feel slightly different.

• mulai besar = start to become big / start getting big
• mulai membesar = also start to get bigger, but with a more explicit verb form

In Indonesian, after mulai, it is very common to use either:

• an adjective: mulai besar
• or a verb: mulai membesar

So mulai besar is natural and efficient. Indonesian often allows adjectives to behave a bit like predicates or states without needing a verb like to be.

Yes. Besar can be used for waves to mean they are big, strong, or rough.

For waves, Indonesian speakers may also say:

• ombaknya besar
• ombaknya tinggi
• gelombangnya besar

So ombak mulai besar is natural. It suggests the sea is becoming rougher.

Indonesian often does not mark plural nouns unless it is necessary.

So ombak can mean:

• a wave
• waves

The context tells you which meaning is intended. Here, because we are talking about sea conditions, English would naturally say the waves even though Indonesian just says ombak.

Indonesian can make plurality clearer by reduplication, for example ombak-ombak, but that is not necessary here and would often sound less natural in this kind of sentence.

Indonesian often shows noun relationships by simply placing two nouns next to each other.

So:

• jangkar kapal = the ship’s anchor / the anchor of the ship

The basic pattern is:

noun + noun

where the second noun often acts like a possessor or classifier.

Other examples:

• pintu rumah = the house door
• mesin mobil = the car engine

So jangkar kapal is completely normal Indonesian structure.

Belum means not yet, while tidak means not.

That difference matters a lot here:

• belum diangkat = it had not been raised yet
• tidak diangkat = it was not raised

Belum suggests the action is expected, possible, or pending. That is exactly what fits an anchor: it has not yet been lifted at that point in time.

Diangkat is the passive form of angkat.

• mengangkat jangkar = to raise the anchor
• jangkar diangkat = the anchor is raised / is being raised / was raised

Indonesian uses passive very naturally, often more naturally than English. Here the focus is on the anchor, not on who is raising it.

If you used an active version, it might be something like:

• awak kapal belum mengangkat jangkar = the crew had not yet raised the anchor

But the original sentence focuses on the state of the anchor, so passive is very suitable.

It comes from the root angkat, which has the basic idea of lift / raise.

Adding di- makes it passive:

• angkat = lift, raise
• diangkat = be lifted, be raised

This di- passive is one of the most important patterns in Indonesian.

Very roughly:

• mengangkat = to raise something
• diangkat = to be raised

Beberapa means some or several.

Once you use beberapa, the idea of plurality is already clear, so the noun does not need any extra plural marking:

• beberapa penumpang = some passengers

This is very typical Indonesian. Compare:

• beberapa buku = several books
• beberapa orang = several people

Reduplication like penumpang-penumpang is not needed here.

Merasa means to feel.

After merasa, Indonesian commonly puts:

• an adjective
• a state
• or a noun phrase describing a condition

So:

• merasa lelah = feel tired
• merasa sakit = feel sick
• merasa mabuk laut = feel seasick

Here mabuk laut is a fixed expression meaning seasick or literally sea-drunk.

You do not need a word like that after merasa in this pattern.

Literally, mabuk can mean drunk or intoxicated, but it is also widely used for motion sickness-type conditions.

So:

• mabuk perjalanan = travel sickness / motion sickness
• mabuk laut = seasickness

In this sentence, mabuk laut clearly means seasick, not alcohol-related drunkenness.

The sentence has a time clause first, then two linked statements:

1. Ketika ombak mulai besar
2. jangkar kapal belum diangkat
3. dan beberapa penumpang merasa mabuk laut

The word dan joins the last two parts.

So structurally, it is:

When X happened, Y happened and Z happened.

This is a very common Indonesian sentence pattern.

Indonesian usually does not mark tense by changing the verb form the way English does.

Instead, time is understood from:

• context
• time words
• aspect words like sudah and belum
• the overall narrative situation

In this sentence, the combination of ketika and belum helps create a clear sequence of events. Even without a past-tense verb form, the sentence is easy to understand as part of a narrative or report about what was happening at that time.

Yes, but the meaning would change slightly.

• beberapa penumpang = some passengers
• para penumpang = the passengers / passengers as a group

So the original sentence specifically says that only some of the passengers felt seasick, not necessarily all of them.

That makes beberapa the better choice here.

Yes. Indonesian allows that.

For example, you could say:

Jangkar kapal belum diangkat dan beberapa penumpang merasa mabuk laut ketika ombak mulai besar.

That is grammatically possible. But putting ketika first is often smoother for setting the scene before describing what happened. It has a natural storytelling feel.