Itu mustahil.

In Indonesian, the verb to be is often left out in simple present-tense sentences.

So:

• Itu mustahil = That/It is impossible
• literally: That impossible

This is completely normal Indonesian. You do not need to add a separate word for is here.

Itu usually means that, but in natural English translation it can also come out as it, depending on context.

For example:

• Itu mustahil = That is impossible
• or It’s impossible

If you are pointing to a specific thing, situation, idea, or claim, itu refers to that thing.

Yes. Mustahil is an adjective meaning impossible.

In Itu mustahil, the structure is:

• itu = that / it
• mustahil = impossible

So the adjective comes after the subject, just like in English: That is impossible.

Yes, Itu mustahil is natural and correct.

Another very common way to express the same idea is:

• Itu tidak mungkin = That’s not possible

The difference is mainly nuance:

• mustahil = impossible, often a bit stronger
• tidak mungkin = not possible, often more everyday and flexible

Both are common.

They are very close, but not always identical in tone.

• mustahil = impossible
  • stronger
  • can sound a little more emphatic or formal
• tidak mungkin = not possible / impossible
  • very common in daily speech
  • often feels slightly more conversational

Examples:

• Itu mustahil. = That’s impossible.
• Itu tidak mungkin. = That’s not possible.

In many situations, either one works.

Yes, itu can sometimes help mark something as definite, a bit like the/that, especially when it comes after a noun:

• buku itu = that book / the book

But in Itu mustahil, itu is not following a noun. It is standing alone as a pronoun, meaning that or it.

So here it is functioning like:

• That is impossible not like
• the impossible

Yes. Ini mustahil means This is impossible.

So:

• Ini mustahil = This is impossible
• Itu mustahil = That is impossible

Use ini for something near you or closer in the conversation, and itu for something farther away or already established.

Usually, no—not as the normal neutral sentence.

The standard order is:

• Itu mustahil

If you say Mustahil! by itself, that is fine and natural as an exclamation:

• Mustahil! = Impossible!

But Mustahil itu would sound unusual unless it appeared in a special poetic or highly marked context.

It is pronounced roughly as:

• moos-TAH-hil or more evenly mus-ta-hil

A simple syllable breakdown is:

• mus-ta-hil

Indonesian stress is usually not as strong or dramatic as in English, so try to pronounce the syllables fairly evenly.

It is not strange or rare, and people do use it in everyday Indonesian. But compared with tidak mungkin, it can sometimes feel a little stronger or slightly more elevated.

So:

• mustahil = perfectly normal
• tidak mungkin = also normal, often especially common in conversation

A learner should understand and be able to use both.