Kalau pintu depan terkunci, kami masuk lewat pintu belakang.

Kalau means if. It introduces a condition:

• Kalau pintu depan terkunci = If the front door is locked

In everyday Indonesian, kalau is very common in conditional sentences. You may also see jika, which also means if, but jika often sounds a bit more formal.

Examples:

• Kalau hujan, saya di rumah. = If it rains, I stay home.
• Jika perlu, saya bantu. = If necessary, I’ll help.

Terkunci means locked.

It comes from the root kunci, which means key or lock, depending on context. With the prefix ter-, it often describes a state or condition. So:

• kunci = key / lock
• terkunci = locked

In this sentence, pintu depan terkunci means the front door is locked.

This is describing the state of the door, not focusing on who locked it.

Not exactly.

• terkunci = locked as a state or condition
• dikunci = locked in the sense of was locked or is locked by someone

Compare:

• Pintu itu terkunci. = The door is locked.
  → Focus on the condition of the door.

• Pintu itu dikunci oleh satpam. = The door was locked by the security guard.
  → Focus on the action and possibly the person who did it.

In your sentence, terkunci sounds very natural because the important idea is simply that the front door is in a locked state.

Indonesian often leaves out the verb to be in sentences like this.

So:

• pintu depan terkunci literally looks like front door locked
• but it means the front door is locked

This is very normal in Indonesian. The language often does not need a separate word for is/am/are when describing something.

More examples:

• Dia sakit. = He/She is sick.
• Rumah itu besar. = That house is big.
• Pintunya terbuka. = The door is open.

This is an important distinction in Indonesian:

• kami = we, but not including the person being spoken to
• kita = we, including the person being spoken to

So in:

• kami masuk lewat pintu belakang

the speaker means we entered/go in through the back door, but the listener is not part of that we.

English only has we, so learners often need to pay special attention to this difference.

Masuk means enter, go in, or come in.

In this sentence:

• kami masuk lewat pintu belakang = we go in / enter through the back door

Depending on context, English may translate it as:

• we enter through the back door
• we go in through the back door
• we come in through the back door

Indonesian does not mark tense the way English does, so masuk by itself does not tell you whether it is present, past, or future. The context decides that.

Here, lewat means through or by way of.

So:

• masuk lewat pintu belakang = enter through the back door

Lewat is very common in everyday speech.
Melalui also means through, but it often sounds a bit more formal or abstract.

Compare:

• Kami masuk lewat pintu belakang. = We enter through the back door.
• Kami masuk melalui pintu belakang. = We enter through the back door.
  → Same meaning, but a bit more formal.

Lewat can also mean to pass by in other contexts:

• Saya lewat toko itu tadi. = I passed by that shop earlier.

In Indonesian, modifiers often come after the noun.

So:

• pintu depan = front door
• pintu belakang = back door

Literally, these are like:

• door front
• door back

This is a common pattern in Indonesian:

• rumah besar = big house
• mobil baru = new car
• kamar mandi = bathroom

So pintu depan is the normal Indonesian order.

Yes, the comma helps show that the first part is the condition and the second part is the result:

• Kalau pintu depan terkunci, kami masuk lewat pintu belakang.

This is similar to English:

• If the front door is locked, we go in through the back door.

In writing, the comma makes the structure clearer. In speech, there is usually a small pause there.

By itself, it is not marked for tense.

Indonesian verbs usually do not change form for tense the way English verbs do. So this sentence could mean different things depending on context:

• If the front door is locked, we go in through the back door.
• If the front door was locked, we went in through the back door.
• If the front door is locked later, we’ll go in through the back door.

Context, time words, or surrounding sentences tell you which time is meant.

For example:

• Kemarin, kalau pintu depan terkunci, kami masuk lewat pintu belakang.
  = Yesterday, if the front door was locked, we entered through the back door.

• Nanti, kalau pintu depan terkunci, kami masuk lewat pintu belakang.
  = Later, if the front door is locked, we’ll go in through the back door.

Yes. You can also put the main clause first and the kalau clause second:

• Kami masuk lewat pintu belakang kalau pintu depan terkunci.

This still means:

• We go in through the back door if the front door is locked.

Both orders are natural. Starting with kalau puts more emphasis on the condition first.