Mulainya kelas musik sering terlambat, jadi kami menunggu sambil bermain catur di lorong.

Mulai is a verb meaning to start / to begin.
Adding -nya turns it into something like a noun phrase: mulainya ≈ its start / the beginning.

• Mulainya kelas musik ≈ the start of the music class
  Literally: its-start the music class.

So mulainya does two things:

1. It nominalizes the verb (start → the start).
2. It gives a sense of definiteness/possession (that particular start, its start).

Saying mulai kelas musik on its own is not natural in this sentence. You would instead say:

• Kelas musik sering mulai terlambat.
  The music class often starts late.

Here mulai is a verb and kelas musik is the subject. Putting mulai in front and then kelas musik right after it (mulai kelas musik sering terlambat) doesn’t fit normal Indonesian syntax.

Starting with Mulainya kelas musik focuses on the beginning itself:

• Mulainya kelas musik sering terlambat...
  The start of the music class is often late...

This structure highlights the starting time as the topic.

You could also say:

• Kelas musik sering mulai terlambat, jadi kami menunggu sambil bermain catur di lorong.

This is very natural too and sounds slightly more straightforward: it treats kelas musik as the topic and mulai terlambat as what it does.

Other possible (more spoken) variants:

• Kelas musik mulainya sering terlambat...
• Mulai kelas musiknya sering terlambat... (colloquial, can sound a bit “loose”)

So the original word order is a stylistic choice that foregrounds the beginning of the class.

Indonesian usually does not use a verb like “is/are” (a copula) before adjectives or stative words.

• sering = often
• terlambat = late (in time)

So:

• Mulainya kelas musik sering terlambat
  literally: Its start of the music class often late.

In English we must say is often late, but in Indonesian terlambat by itself acts as a predicate (to be late). Sering modifies terlambat, just like often modifies late.

So the hidden structure is:

• Mulainya kelas musik [Ø] sering terlambat
  ≈ The start of the music class [is] often late.

• lambat = slow (about speed)
• terlambat = late (about time; too late)

Examples:

• Mobil itu sangat lambat.
  That car is very slow.

• Dia datang terlambat.
  He/She came late.

Because the sentence is about the starting time of the class relative to a schedule, terlambat (late) is the correct choice, not lambat (slow).

Yes, in this sentence jadi works like the connector so / therefore:

• ..., jadi kami menunggu sambil bermain catur di lorong.
  ..., so we wait while playing chess in the hallway.

Some notes:

• jadi is very common in speech and neutral in tone.
• In more formal writing, people often prefer sehingga, oleh karena itu, or akibatnya:
  • ..., sehingga kami menunggu sambil bermain catur di lorong.

Be aware jadi can also mean to become / to turn into in other contexts (e.g. Dia jadi guru. – He became a teacher), but here it’s clearly a conjunction meaning so.

Indonesian distinguishes two kinds of we:

• kami = we (but NOT you) → exclusive
• kita = we (including you) → inclusive

In the sentence:

• ..., jadi kami menunggu sambil bermain catur di lorong.

kami suggests the speaker is talking about their own group, not including the listener (maybe “we students in that class”).

If the speaker is talking to someone who is also part of that same group (for example, a classmate in the same music class), they could use:

• ..., jadi kita menunggu sambil bermain catur di lorong.

So:

• kami = describing our habit to an outsider.
• kita = including the listener in that experience.

sambil means while (at the same time as) and indicates two actions happening simultaneously:

• menunggu sambil bermain catur
  ≈ wait while playing chess
  → Waiting is the main activity; playing chess happens alongside it.

If you said:

• menunggu dan bermain catur

it just means to wait and play chess (a simple list), without clearly saying that they are done at the same time. It could sound more like two separate activities.

So sambil is used when:

• there is a main action (menunggu),
• plus another action happening concurrently (bermain catur).

Yes, that is natural and grammatical:

• Sambil bermain catur di lorong, kami menunggu.

This keeps the same meaning: While playing chess in the hallway, we wait.

Differences in nuance:

• Kami menunggu sambil bermain catur di lorong.
  → starts with kami menunggu, focusing first on the fact that we wait.
• Sambil bermain catur di lorong, kami menunggu.
  → starts with the “while playing chess” part, placing a bit more emphasis on the side activity or the situation.

Both are fine in everyday Indonesian.

• di = in / at / on (location preposition)
• lorong = hallway / corridor / passage

So di lorong = in the hallway.

In Indonesian, location phrases are flexible in position. You can say:

1. ... kami menunggu sambil bermain catur di lorong.
   (neutral, very common: place information at the end)

2. ... kami menunggu di lorong sambil bermain catur.
   (still natural; small change in rhythm)

3. Di lorong, kami menunggu sambil bermain catur.
   (fronted for emphasis on the location.)

All are grammatical; you just slightly change what you foreground (activity vs. place).

Common related words:

• lorong
  General word for a hallway, corridor, passage (inside a building or other narrow passage).

• koridor
  Loanword from corridor, often used for building corridors too. Sounds a bit more formal/technical or associated with larger buildings.

• gang
  A narrow alley or lane outside between houses, not inside a building. So gang is not appropriate for a school hallway.

In a school context, both:

• lorong and
• koridor

can work; lorong is more common and neutral in everyday speech.

Indonesian verbs do not inflect for tense. Time is understood from context and from time-related words.

Here:

• sering = often → suggests a habitual action.
• So the whole sentence describes a repeated, habitual situation.

Depending on context, it could be:

• present habit: The start of the music class is often late, so we (usually) wait...
• past habit: Back then, the start of the music class was often late, so we waited...

If you want to make it explicitly past, you can add markers:

• Dulu, mulainya kelas musik sering terlambat, jadi kami menunggu sambil bermain catur di lorong.
  In the past / back then, the start of the music class was often late...

For future:

• Nanti mulainya kelas musik mungkin terlambat, jadi kami akan menunggu sambil bermain catur di lorong.
  Later the start of the class might be late, so we will wait...

That version is not natural.

• mulai by itself is a verb (to start).
• To mean the start (of something), Indonesian typically uses mulai
  • -nya → mulainya.

So:

• Mulainya kelas musik sering terlambat... ✔️
  The start of the music class is often late...

But:

• Mulai kelas musik sering terlambat... ✖️
  feels ungrammatical/awkward, because mulai as a verb shouldn’t directly precede kelas musik like that in this structure.

If you want mulai as a verb, use:

• Kelas musik sering mulai terlambat... ✔️
  The music class often starts late...

sedang marks an activity as in progress right now (similar to the English present continuous “am/is/are -ing”).

• Kami menunggu sambil bermain catur.
  → can be a general habit or a description of what happens when class is late.

• Kami sedang menunggu sambil bermain catur.
  → We are currently waiting while playing chess – focusing on right now.

In your original sentence, because sering makes it a habitual statement (this often happens), using sedang would be strange; it would mix right now with often.

So for general habits, you normally omit sedang:

• Mulainya kelas musik sering terlambat, jadi kami menunggu sambil bermain catur di lorong. ✔️

The sentence is neutral–informal:

• jadi is common in spoken and informal written Indonesian.
• The rest of the vocabulary is neutral.

For a school essay or more formal writing, many teachers would still accept it, but a slightly more formal connector could be preferred:

• Mulainya kelas musik sering terlambat, sehingga kami menunggu sambil bermain catur di lorong.

Changing jadi to sehingga makes it sound more formal/academic, but the original is perfectly fine for everyday use and semi-formal contexts.

• kelas musik
  Can mean either:

  • a class session (the period), or
  • a class/group of students learning music.
    It’s widely used and understood.

• pelajaran musik
  Literally music lesson/subject.
  Often used in formal school contexts to refer to the subject in the curriculum.

• kursus musik
  music course, often a private course or lessons outside regular school (e.g., at a music school or institute).

In your sentence:

• Mulainya kelas musik sering terlambat...

it sounds like talking about the scheduled music class at school. You could also say pelajaran musik with nearly the same meaning if you want to emphasize it as a part of the school curriculum:

• Mulainya pelajaran musik sering terlambat...