Di dalam bas awam, ramai orang memakai topeng ketika musim jerebu.

• dalam = inside / in
• di = a general preposition meaning at / in / on
• di dalam together literally means at the inside (of), and is often used like in / inside.

In this sentence, Di dalam bas awam is completely natural. You can also say:

• Dalam bas awam, ramai orang memakai topeng...

Both are grammatically correct.

Nuance (often very slight, and context‑dependent):

• di dalam can sound a bit more explicit or slightly more formal/complete.
• dalam alone is a bit more neutral and very common in everyday speech.

In everyday conversation, you will hear both, and for most purposes you can treat them as interchangeable here.

In Malay, descriptive words (adjectives) usually come after the noun:

• bas awam = public bus (literally: bus public)
• rumah besar = big house (literally: house big)
• telefon baharu = new phone (literally: phone new)

So:

• bas = bus
• awam = public

You would not say awam bas in standard Malay; that order is wrong. The normal pattern is:

Noun + Adjective
bas awam, kereta merah, baju hijau, etc.

Awam can mean public in several related senses:

1. Public / general population

   • orang awam = the public / civilians / ordinary people

2. Public (not private), shared by everyone

   • pengangkutan awam = public transport
   • bas awam = public bus
   • tandas awam = public toilet

In bas awam, awam means the bus is for general public use (not a private or chartered bus). It’s the same kind of public as in public transport.

Both ramai and banyak relate to many / a lot of, but:

• ramai is normally used for people.
• banyak is used for things, animals, abstract stuff, etc.

Examples:

• ramai orang = many people

• ramai pelajar = many students

• banyak kereta = many cars
• banyak masaalah = many problems
• banyak anjing = many dogs

So ramai orang is the natural choice here.
Banyak orang is understandable but sounds non‑native or wrong in standard Malay.

Malay often does not mark plural explicitly; context tells you whether it’s singular or plural.

• orang can mean person or people, depending on context.
• ramai before orang clearly indicates plurality: ramai orang = many people.

Reduplication (repeating the noun) like orang-orang can show plurality, but:

• It is not required here because ramai already shows “many”.
• orang-orang is more often used for specific groups, or stylistically, or in certain set phrases.

So:

• ramai orang = natural, normal.
• ramai orang-orang = grammatically possible but sounds awkward and is almost never said.

Both come from the same root pakai (to use / to wear).

• pakai = base verb, very common in informal and neutral speech.
• memakai = the meN- (here mem-) verb form, often a bit more formal or bookish, and common in writing.

In speech:

• ramai orang pakai topeng – very natural in everyday conversation.

In writing or more formal contexts:

• ramai orang memakai topeng – sounds appropriate and slightly more formal.

Meaning-wise here, they are the same: to wear (a mask, clothes, etc.). As a learner, you can safely use pakai in most conversations and recognize memakai in texts.

Yes, memakai/pakai can mean both to wear and to use, depending on the object:

• memakai topeng = wear a mask
• memakai baju = wear clothes

But also:

• memakai kaedah ini = use this method
• pakai cara lama = use the old way

Context decides whether you translate it as wear or use. In this sentence, memakai topeng clearly means wear masks.

All can be translated roughly as when / while, but with different typical uses and levels of formality:

• ketika

  • Often means when / at the time when, sometimes with a sense of a period.
  • Neutral, OK in spoken and written Malay.
  • Fits well with musim (season/time): ketika musim jerebu = during the haze season.

• semasa

  • Very close to ketika, often interchangeable here.
  • Also used for during / while: semasa musim jerebu.

• apabila

  • More formal, often used when talking about specific events or conditions:
    
    • apabila hujan turun = when it rains.

• bila

  • More informal / conversational; in many dialects it’s the everyday word for when.
  • Example: bila musim jerebu? = when is the haze season?

In this sentence, ketika musim jerebu = during the haze season / when it’s haze season.
You could also say semasa musim jerebu with almost no change in meaning.

Literally:

• musim = season / period
• jerebu = haze (especially air pollution haze from smoke, etc.)

So musim jerebu = haze season or the haze period (the time of year when haze is common).

In English you might translate the whole phrase more naturally as:

• during the haze season
• during the hazy period
• when it’s haze season

You could also say in Malay:

• semasa jerebu = during haze / when it’s hazy (more general, not specifically a “season”).

The comma after Di dalam bas awam is optional and mainly reflects a pause in speech, like in English:

• Di dalam bas awam, ramai orang memakai topeng...
  = On public buses, many people wear masks...

Structurally, Di dalam bas awam is a prepositional phrase giving the setting (where). Placing it at the start is common, and using a comma afterwards helps readability, but it’s not a strict grammar rule.

You could also write:

• Di dalam bas awam ramai orang memakai topeng ketika musim jerebu.

Both are acceptable; the meaning is the same.

Malay verbs do not change form for tense. Memakai is the same for:

• wear / are wearing / wore / will wear, etc.

Tense is understood from context or from optional time words:

• semalam = yesterday
• sekarang = now
• nanti / akan = later / will

For example:

• Semalam di dalam bas awam, ramai orang memakai topeng.
  = Yesterday on public buses, many people wore masks.

• Sekarang di dalam bas awam, ramai orang memakai topeng.
  = Now on public buses, many people are wearing masks.

In your sentence, without any time word, it’s usually translated in a general present sense:

On public buses, many people wear masks during the haze season.

Yes, you can say di bas, and people do, but there are nuances:

• di bas = on/at the bus (more general, could be inside or just at the bus area).
• di dalam bas = explicitly inside the bus.

Common patterns:

• Saya tunggu di bas.
  = I wait at/by the bus. (context might be stop/terminal)

• Saya duduk di dalam bas.
  = I sit in the bus (inside it).

In everyday speech, many speakers shorten di dalam bas to just dalam bas or even just dalam bas without di:

• Dalam bas, ramai orang pakai topeng.

All of these are understandable; di dalam bas is clear and slightly more explicit that you mean inside the bus.