Bumi sangat besar.

Because Bumi here refers to the planet Earth, which is treated like a proper name.

• Bumi = Earth the planet
• bumi in lowercase can also mean earth, land, or ground in some contexts

So the capital letter helps show that this is the planet, not just soil or land.

Indonesian often does not use a verb like is / am / are in simple sentences with nouns or adjectives.

So:

• Bumi sangat besar. = Earth is very big.

Literally, Indonesian is more like:

• Earth very big.

This is completely normal and grammatical in Indonesian.

The pattern is:

• Bumi = subject
• sangat besar = predicate

So the sentence structure is:

• Subject + description

In English, you need is:

• The Earth is very big.

In Indonesian, you can just put the description after the subject:

• Bumi sangat besar.

Sangat means very.

It usually comes before the adjective or adverb it modifies:

• sangat besar = very big
• sangat kecil = very small
• sangat cepat = very fast

So sangat besar is the normal order, not besar sangat in standard Indonesian.

Yes. That is also natural.

Both of these mean very big:

• sangat besar
• besar sekali

A rough difference:

• sangat besar = straightforward very big
• besar sekali = also very big, sometimes with a slightly stronger or more emphatic feel

So:

• Bumi sangat besar.
• Bumi besar sekali.

Both are correct.

In this sentence, besar is part of the predicate, so it comes after the subject Bumi.

More generally, Indonesian often places descriptive words after the noun:

• rumah besar = big house
• kota besar = big city

And in full sentences, a subject is followed by its description:

• Bumi sangat besar.

So this word order is very normal in Indonesian.

No. Besar often means big or large, but it can also mean things like major, important, or serious, depending on context.

Examples:

• rumah besar = big house
• kota besar = big city
• masalah besar = big/serious problem

In Bumi sangat besar, it clearly refers to physical size.

Not usually. Indonesian does not have articles like a, an, and the in the same way English does.

So Bumi by itself is enough to mean Earth or the Earth, depending on context.

That is why Indonesian says:

• Bumi sangat besar.

where English says:

• The Earth is very big.

Because Bumi specifically refers to Earth as a planet.

Dunia usually means the world in a broader sense, such as:

• human society
• worldly life
• the world in general

So:

• Bumi sangat besar. = about the planet Earth
• Dunia would not be the best choice if you specifically mean the astronomical planet

A simple English-friendly guide is:

• Bumi ≈ BOO-mee
• sangat ≈ SAH-ngaht
• besar ≈ buh-SAR

A few notes:

• u in Bumi sounds like oo in food
• ng in sangat is like the sound in singer
• the r in besar is usually tapped or rolled lightly

So the whole sentence is roughly:

• BOO-mee SAH-ngaht buh-SAR