Jembatan itu dibangun satu abad lalu.

In Indonesian, demonstratives like ini and itu usually come after the noun.

So:

• jembatan itu = that bridge
• rumah ini = this house

This is normal Indonesian word order.
It can also sometimes feel like the bridge in English, depending on context, not only that bridge.

It can mean either, depending on context.

• that bridge if you are pointing it out or contrasting it with another bridge
• the bridge if both speaker and listener already know which bridge is being discussed

Indonesian often uses itu in places where English would simply use the.

Dibangun is the passive form, while membangun is the active form.

• membangun = to build
• dibangun = to be built / was built

So:

• Mereka membangun jembatan itu. = They built the bridge.
• Jembatan itu dibangun... = The bridge was built...

In your sentence, the focus is on the bridge, not on who built it, so the passive is the natural choice.

The prefix di- is a common marker of the passive voice in Indonesian.

So:

• membangun = active, to build
• dibangun = passive, built / was built

In this sentence, di- tells you that the bridge receives the action.

A useful pattern is:

• Rumah itu dibangun tahun lalu. = The house was built last year.
• Buku itu ditulis oleh guru saya. = The book was written by my teacher.

The base is bangun.

In this context:

• membangun = to build
• dibangun = to be built

Be aware that bangun can also have another common meaning related to waking up / getting up. Indonesian roots often have more than one related use, so context matters.

In Jembatan itu dibangun..., because the subject is jembatan, the meaning is clearly about construction, not waking up.

Indonesian does not change the verb the way English does for tense.
Instead, time is usually shown by time expressions.

Here, the past meaning comes from satu abad lalu:

• satu abad = one century
• lalu = ago

So the sentence is understood as past because it says one century ago.

This is very common in Indonesian:

• Saya makan tadi pagi. = I ate this morning.
• Mereka datang kemarin. = They came yesterday.

The verb itself stays the same.

It literally means one century ago.

Breakdown:

• satu = one
• abad = century
• lalu = ago

So satu abad lalu is a time phrase meaning a hundred years ago.

English often says a century ago, while Indonesian commonly says satu abad lalu.

Yes. Seabad lalu is also correct and very natural.

• satu abad lalu = one century ago
• seabad lalu = a century ago

The prefix se- can mean one in expressions like this.

So both are acceptable, though seabad lalu may sound a bit more compact and idiomatic.

In Indonesian passive sentences, the agent is often left out if it is unknown, obvious, or unimportant.

So:

• Jembatan itu dibangun satu abad lalu. = The bridge was built a century ago.

If you want to mention who did it, you can add oleh:

• Jembatan itu dibangun oleh Belanda satu abad lalu. = The bridge was built by the Dutch a century ago.

But Indonesian often omits the agent more readily than English does.

Yes. Both are possible.

• satu abad lalu
• satu abad yang lalu

Both mean one century ago.

The version without yang is shorter and very common.
The version with yang can sound slightly more explicit or formal, but the meaning is the same.

Yes, to some extent.

The original sentence:

• Jembatan itu dibangun satu abad lalu.

You could also say:

• Satu abad lalu, jembatan itu dibangun.

This changes the emphasis a little. The second version foregrounds the time expression one century ago.

However, the original order is very natural and neutral.

Yes, it is completely natural.

It is a straightforward passive sentence with a time expression:

• Jembatan itu = the bridge / that bridge
• dibangun = was built
• satu abad lalu = a century ago

It sounds like something you might read in a history text, travel guide, or museum description.

An active version would need a subject that performs the action.

For example:

• Mereka membangun jembatan itu satu abad lalu. = They built the bridge a century ago.
• Pemerintah kolonial membangun jembatan itu satu abad lalu. = The colonial government built the bridge a century ago.

So the difference is:

• active: focuses on the builder
• passive: focuses on the bridge

That is why the passive version is especially useful when the builder is unknown or not important.