Webinar itu diikuti oleh banyak peserta.

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

So:

• webinar itu = that webinar / the webinar
• peserta itu = that participant

This is different from English, where that comes before the noun.

In many contexts, itu can also make the noun feel definite, so webinar itu can mean something like the webinar if both speaker and listener already know which webinar is being talked about.

Diikuti here means was followed/joined/attended, depending on context.

In this sentence, the most natural English meaning is:

• The webinar was attended by many participants.

The verb comes from:

• ikut = to join / take part
• mengikuti = to follow, attend, take part in
• diikuti = to be followed / attended / taken part in

With webinar, the best translation is usually attended.

No. In diikuti, di- is not the preposition di meaning in/at/on.

Here, di- is a passive verb prefix, so it must be attached to the verb:

• diikuti = passive verb

Compare:

• di rumah = at home → di is a preposition, so it is written separately
• diikuti = was attended/followed → di- is a prefix, so it is written together

This is a very common point of confusion for learners.

The sentence is passive because the thing being talked about first is the webinar, not the participants.

• Webinar itu diikuti oleh banyak peserta. = That webinar was attended by many participants.

Indonesian uses passive very naturally, often more often than English does. It is especially common when the speaker wants to focus on the object/event rather than the doer.

The active version would be:

• Banyak peserta mengikuti webinar itu. = Many participants attended that webinar.

Both are correct, but the focus is different:

• Passive: focus on the webinar
• Active: focus on the participants

Oleh means by and marks the agent in a passive sentence.

So:

• diikuti oleh banyak peserta = attended by many participants

It tells you who performed the action.

In Indonesian, oleh is sometimes omitted, especially when the sentence is less formal or the agent is obvious:

• Webinar itu diikuti banyak peserta.

This still means the same thing and sounds natural. Including oleh can sound a bit more formal or explicit.

They are related, but they are not used in exactly the same way.

• ikut often means join, come along, or take part
• mengikuti often means follow, attend, or take part in something

Examples:

• Saya ikut. = I’m joining / I’ll come along.
• Saya mengikuti webinar itu. = I attended/followed that webinar.

With events like classes, seminars, and webinars, mengikuti is very common.

So in your sentence, diikuti is the passive form of mengikuti.

Indonesian nouns do not usually change form for plural the way English nouns do.

So:

• peserta can mean participant or participants, depending on context

In this sentence, plurality is already shown by banyak:

• banyak peserta = many participants

Because banyak already makes it plural, there is no need to repeat plurality in another way.

Yes, literally followed is close to the structure of the Indonesian verb, but in natural English, attended is better here.

For a webinar, English usually says:

• attend a webinar
• join a webinar

So the best natural translation is:

• That webinar was attended by many participants.

Literal translation can help you see the structure, but natural translation is better for actual usage.

Yes. Webinar is widely used in Indonesian and is completely normal in modern usage.

You may also see more formal or less borrowed expressions, but webinar is extremely common in everyday Indonesian, especially in education, business, and online events.

So this sentence sounds natural.

Yes, absolutely.

• Webinar itu diikuti oleh banyak peserta.
• Webinar itu diikuti banyak peserta.

Both are correct.

The version without oleh is often more direct and still very natural. The version with oleh is a little more explicit and can sound slightly more formal.

Because banyak peserta already clearly means many participants.

Using para together with banyak is usually unnecessary and often sounds odd. In general:

• peserta = participant(s)
• para peserta = the participants
• banyak peserta = many participants

So banyak peserta is the best choice here.

The structure is:

• Webinar itu = topic / subject being discussed
• diikuti = passive verb
• oleh banyak peserta = agent phrase (by many participants)

So the pattern is:

• [Thing/Event] + passive verb + oleh + doer

This is a very useful Indonesian pattern to recognize.

Another example:

• Buku itu dibaca oleh siswa-siswa. = The book was read by the students.

Yes, it has basically the same core meaning, but the emphasis changes.

• Webinar itu diikuti oleh banyak peserta. = focus on the webinar
• Banyak peserta mengikuti webinar itu. = focus on the participants

So the event is the same, but the sentence presents it from a different angle. This is one reason passive is so common in Indonesian: it lets you put the most important thing first.