Tadi malam kamar saya kemasukan nyamuk karena jendelanya tidak tertutup rapat.

Yes. Tadi malam commonly means last night.

• tadi = earlier, just now, earlier today/in the recent past
• malam = night

Together, tadi malam means last night or earlier last night.

A learner should know that tadi by itself often refers to something recent, but in combinations like tadi pagi / tadi siang / tadi malam, it points to a specific earlier time period:

• tadi pagi = this morning / earlier this morning
• tadi siang = earlier this afternoon
• tadi malam = last night

Because Indonesian usually puts the possessor after the noun.

• kamar saya = my room
• literally: room my

This is the normal pattern:

• buku saya = my book
• rumah dia = his/her house
• teman mereka = their friend

So kamar saya is the standard way to say my room.

Kemasukan here means something like to get entered by, to have something come into it, or more naturally in English, to have something get inside.

So:

• kamar saya kemasukan nyamuk = my room got mosquitoes in it / mosquitoes got into my room

This verb has a useful nuance: it often suggests that something unwanted or unexpected got inside.

Examples:

• Rumah saya kemasukan pencuri. = My house was broken into by a thief / thieves.
• Mata saya kemasukan debu. = Dust got into my eye.
• Kamar saya kemasukan nyamuk. = Mosquitoes got into my room.

So kemasukan is not just about physical entry; it often carries the idea that something got in and caused a problem.

It comes from the root masuk = to enter / go in, with the circumfix ke- -an:

• masuk = enter, go in
• ke + masuk + an → kemasukan

In many cases, ke- -an can show that the subject experiences something, often something accidental, uncontrolled, or unpleasant.

In this sentence:

• kamar saya kemasukan nyamuk
• literally: my room experienced mosquitoes entering it

That is why kemasukan feels different from just dimasuki or ada nyamuk di kamar saya.

You can say that, but the meaning and emphasis are a bit different.

• Ada nyamuk di kamar saya. = There are mosquitoes in my room.
• Kamar saya kemasukan nyamuk. = My room got mosquitoes in it / mosquitoes got into my room.

The first sentence is more neutral: it simply states that mosquitoes were there.

The second sentence emphasizes:

• entry
• something unwanted
• often a sense of cause or accident

Because the sentence continues with karena jendelanya tidak tertutup rapat, kemasukan fits very well: the mosquitoes got in because the window was not tightly closed.

In Indonesian, nouns often do not have to be marked for singular or plural. So nyamuk can mean:

• a mosquito
• mosquitoes

The context tells you which meaning is intended.

Here, English would naturally translate it as mosquitoes, because:

• that is the most likely real-life meaning
• kemasukan nyamuk often implies mosquitoes got in, not just one

If Indonesian wants to make plurality very explicit, it can use reduplication:

• nyamuk-nyamuk = mosquitoes

But in normal speech, that is often unnecessary.

Karena means because.

It introduces the reason:

• karena jendelanya tidak tertutup rapat
• because the window wasn’t tightly closed

So the sentence structure is:

• Tadi malam = time
• kamar saya kemasukan nyamuk = main event
• karena jendelanya tidak tertutup rapat = reason

This is very straightforward Indonesian word order.

Jendelanya means the window, his/her window, its window, or sometimes the window in question, depending on context.

Here, -nya is best understood as making the noun more specific:

• jendela = window
• jendelanya = the window / its window / the window of it

In this sentence, it most naturally refers to the room’s window:

• kamar saya ... karena jendelanya ...
• my room ... because its/the window ...

So -nya does not always mean his/her. It can also function like the or the relevant one in context.

Both are possible, but they sound a bit different.

• jendelanya = the window / its window
• jendela kamar saya = my room’s window / the window of my room

Using jendelanya is more natural and efficient here because the room has already been mentioned. Once kamar saya is established, jendelanya smoothly refers back to the window belonging to that room.

This kind of reference is very common in Indonesian.

Tertutup means closed.

It comes from:

• tutup = lid, cover; to close
• tertutup = closed

The prefix ter- here often describes a state or condition:

• pintu tertutup = the door is closed
• jendela tertutup = the window is closed

So:

• jendelanya tidak tertutup rapat = the window was not tightly closed

A useful simple distinction is:

• tutup can be a root meaning close or cover
• tertutup describes the state of being closed

Compare:

• Saya menutup jendela. = I close the window.
• Jendelanya tertutup. = The window is closed.

So in your sentence, tertutup is appropriate because it describes the condition of the window, not the action of someone closing it.

Here rapat means tightly, firmly, or completely shut.

So:

• tertutup = closed
• tertutup rapat = tightly closed / shut properly / sealed shut

This is a very common collocation in Indonesian.

Examples:

• Pintunya tertutup rapat. = The door is tightly shut.
• Botol itu ditutup rapat. = The bottle was closed tightly.

In your sentence, tidak tertutup rapat means the window was closed, or partly closed, but not tightly enough, which allowed mosquitoes to enter.

Because tertutup is a verb/adjectival predicate, and tidak is the normal negator for verbs and adjectives.

• tidak negates verbs and adjectives
• bukan usually negates nouns, noun phrases, or identification

So:

• jendelanya tidak tertutup rapat = the window was not tightly closed

Compare:

• Dia tidak sakit. = He/she is not sick.
• Itu bukan rumah saya. = That is not my house.

Using bukan tertutup here would be incorrect.

Yes, it is very natural.

Indonesian often puts a time expression at the beginning of the sentence:

• Tadi malam = last night
• kemarin = yesterday
• besok pagi = tomorrow morning

So this structure is very common:

• [time] + [main clause] + [reason]

In this sentence:

• Tadi malam = last night
• kamar saya kemasukan nyamuk = my room got mosquitoes in it
• karena jendelanya tidak tertutup rapat = because the window wasn’t tightly closed

Starting with the time helps set the scene immediately.

That is close to the structure, but it sounds unnatural in English.

A more natural English translation would be:

• Last night, mosquitoes got into my room because the window wasn’t tightly closed.
• Last night, my room got mosquitoes in it because the window wasn’t shut tightly.

The literal wording helps you understand the grammar, but natural translation should usually focus on how English actually says it.

Yes, indirectly. It refers back to something already mentioned, in this case kamar saya.

So jendelanya can be understood as:

• the room’s window
• its window
• naturally in English: the window

Even though -nya often gets taught as his/her, in real Indonesian it is much broader. It can refer to:

• a previously mentioned thing
• a known item in context
• something like the

That is exactly what is happening here.