Learn Hardcore Indonesian: Setelah mengecat, kami membuka jendela supaya udara cepat masuk. - After painting, we open the window so air comes in quickly.

mengecat means to paint a surface with paint (walls, doors, fences). It’s the normal verb derived from the noun cat (paint).
melukis means to paint as in creating artwork (to paint a picture).
mencat is nonstandard in modern Indonesian; the accepted form is mengecat.

How is the verb mengecat formed?

It’s the prefix meN- (active verb) added to the base cat. With monosyllabic bases like cat, the allomorph menge- is used: menge + cat → mengecat. Similar patterns: mengebor (from bor, to drill), mengepel (from pel, to mop), mengecek (from cek, to check; colloquial).

Why is the subject omitted in Setelah mengecat?

Indonesian can drop the subject in a dependent clause when it’s the same as the main clause subject. It’s understood that the subject of mengecat is kami. If you need to be explicit or if the subject is different, say:

Setelah kami mengecat, …
Setelah dia mengecat, kami membuka jendela.

Can I change the order of the clauses?

Yes. All of these are fine:

Setelah mengecat, kami membuka jendela…
Kami membuka jendela setelah mengecat…
Setelah kami mengecat, kami membuka jendela… (more explicit)

Is the comma after Setelah mengecat required?

Yes. When an adverbial clause (like one with setelah) comes first, standard punctuation uses a comma before the main clause. If the adverbial clause comes after, you normally don’t use a comma: Kami membuka jendela setelah mengecat.

What’s the difference among supaya, agar, and biar?

They all introduce a purpose/result clause meaning so that/in order that:

supaya: neutral, common
agar: a bit more formal
biar: informal/colloquial All work here: supaya/agar/biar udara cepat masuk.

Can I use untuk instead of supaya?

Not with a full clause like this. Untuk typically precedes a noun/gerund or a verb phrase that functions like a noun:

Natural: Kami membuka jendela untuk mempercepat pengeringan (cat).
Not natural: ✗ … untuk udara cepat masuk. Use supaya/agar/biar + clause when the next part has its own subject and verb.

Why is it udara and not angin?

udara = air (general; the substance)
angin = wind/breeze (moving air) Both can fit, but udara is neutral and common in this purpose clause. Angin would highlight the breeze: … supaya angin cepat masuk.

Does cepat act as an adverb here? Could I say dengan cepat?

Yes, cepat can function adverbially. You can also say dengan cepat:

… supaya udara cepat masuk.
… supaya udara masuk dengan cepat. For a comparative: … supaya udara masuk lebih cepat (so the air comes in faster).

Why is it masuk and not memasukkan?

masuk is intransitive (to enter/come in). The subject is udara (air) which is entering by itself: udara masuk. memasukkan is transitive (to put/bring something into something): memasukkan udara would mean someone is inserting air (e.g., inflating a tire), which isn’t the meaning here.

Do I need ke after masuk?

Only if you name the destination. With no destination, masuk stands alone:

Plain: udara cepat masuk
With destination: udara cepat masuk ke dalam ruangan/kamar.

Is membuka mandatory, or can I use buka?

In standard style, use membuka for the active transitive verb. In everyday speech, you’ll also hear buka used as a bare verb: Kami buka jendela… Both are understood; membuka is more formal/neutral.

How do I show singular vs plural for jendela?

Indonesian has no articles, so jendela can be the/a window or windows, depending on context. To be explicit:

One window: sebuah jendela
Some windows: beberapa jendela
All windows: semua jendela
Plural by reduplication: jendela-jendela
Definite (context-known): jendelanya (the window[s])

How is past time shown? There’s no past tense marking.

Correct—Indonesian doesn’t inflect for tense. Sequence words like setelah (after) place the actions in time. To be more explicit, add time adverbs:

tadi (earlier today), barusan (just now), kemarin (yesterday), etc. Example: Kemarin, setelah mengecat, kami membuka jendela…

Could I add bisa in the purpose clause (supaya udara bisa cepat masuk)?

You can. bisa adds the nuance of possibility/ability: so that the air can quickly come in. It’s optional here; supaya udara cepat masuk is already natural.

If I want to emphasize finishing the painting, how can I say that?

Use selesai or a passive:

Setelah selesai mengecat, kami membuka jendela… (After finishing painting…)
Setelah (semuanya) dicat, kami membuka jendela… (After it was painted…)