Learn Hardcore Malay: Kami masuk melalui pintu belakang.

kami = we/us, excluding the listener. Use it when the person you’re talking to is not part of the group.
kita = we/us, including the listener. Use it when the person you’re talking to is part of the group. Examples:
Kami masuk melalui pintu belakang. = We (not you) went in through the back door.
Kita masuk melalui pintu belakang. = We (you and I) go in through the back door.

Does the sentence show past, present, or future?

Malay verbs don’t change for tense. Kami masuk melalui pintu belakang can mean past, present, or future depending on context. To make it explicit:

Past: Kami sudah/telah/dah masuk melalui pintu belakang.; add time words like tadi, semalam.
Present progressive: Kami sedang masuk melalui pintu belakang. (possible, though the act of entering is brief).
Future: Kami akan masuk melalui pintu belakang.; or use nanti, sebentar lagi.

Do I need ke after masuk, like masuk ke?

With destinations, ke is optional: masuk rumah ≈ masuk ke rumah. Ke dalam adds “into the inside”: masuk ke dalam rumah.
Don’t say masuk ke pintu (you don’t enter into a door). You enter a place through a door: Kami masuk ke dalam rumah melalui pintu belakang.

What’s the difference between masuk and memasuki?

masuk is intransitive or followed by a prepositional phrase: Kami masuk melalui pintu belakang.
memasuki is a transitive verb meaning “to enter (something)” and is more formal: Kami memasuki rumah melalui pintu belakang. Formally, memasuki = meN- + masuk
- -i.

Can I use something other than melalui to say “through/via”?

Yes, depending on register:

Standard/formal: melalui (very common), menerusi (also formal).
Colloquial: ikut (via/by way of): Kami masuk ikut pintu belakang.
Many speakers also say dari: Kami masuk dari pintu belakang (common, though purists prefer melalui).
Indonesian note: lewat = via; in Malay, lewat mainly means “late.”

Can masuk mean “to join,” and does that affect this sentence?

Masuk can mean “to join/enter (an institution/group): masuk universiti, masuk kelab. Your sentence with melalui pintu belakang is literal (physically through a door). With a different object, it can be figurative: Dia masuk universiti melalui pintu belakang = he got in by “backdoor” means (improperly).

Does pintu belakang have an idiomatic meaning?

Yes. Literally “back door,” but in politics/admissions it suggests illegitimate/underhanded entry, e.g. kerajaan pintu belakang (backdoor government) or masuk universiti melalui pintu belakang (admitted by improper means). In ordinary house contexts, it’s just the literal back door.

What’s the difference between pintu belakang, pintu di belakang, and di belakang pintu?

pintu belakang = the back door (a specific door).
pintu di belakang = the door at the back (emphasizes location; often equivalent in context).
di belakang pintu = behind the door (the space posterior to the door), not the door itself. Example: Kunci ada di belakang pintu. = The key is behind the door.

Why is there no “the” or “a” before pintu? How do I make it definite or indefinite?

Malay has no articles. Context supplies definiteness. To mark it:

Definite: pintu belakang itu/ini (that/this back door).
Indefinite/one unit: sebuah pintu belakang (a back door/one back door).

Can I change the word order for emphasis or drop the subject?

Default: Kami [S] masuk [V] melalui pintu belakang [PP].
Fronting for emphasis (more formal/written): Melalui pintu belakang, kami masuk.
Subject drop (when context is clear, especially spoken): Masuk melalui pintu belakang.

How do I pronounce the tricky parts?

melalui: [mə-la-lu-i]; the e is a schwa; lu-i has a vowel hiatus (both vowels pronounced).
belakang: [bə-LA-kang]; final ng is the velar nasal [ŋ], not an English “ng+k.”
Stress is light and even; don’t over-stress syllables as in English.

How do I negate this or turn it into a question?

Negation: Kami tidak masuk melalui pintu belakang. (standard) / Kami tak masuk… (colloquial).
Negative imperative: Jangan masuk melalui pintu belakang.
Yes–no question: Use rising intonation: Kami masuk melalui pintu belakang?; or formal Kami masuk melalui pintu belakangkah? / Adakah kami masuk melalui pintu belakang?

How do I say how many people are in kami?

Add a numeral classifier phrase:

Kami berdua/bertiga/berempat … = The two/three/four of us …
You can also say kami semua to emphasize “all of us.”

If it’s not a literal door, how do I say “We went in from the back (side)”?

Use a general route/location phrase:

Kami masuk dari belakang.
Kami masuk dari arah belakang.
Kami masuk melalui bahagian belakang. These mean “from the back side,” without implying a physical door.