Rancangan ini mustahil.

Malay often leaves out the verb to be in simple statements.

So:

• Rancangan ini mustahil = This plan is impossible

Malay does not need a separate word like is here. This is very normal in equational or descriptive sentences.

You could sometimes add words like adalah or ialah, but not in this sentence in normal everyday usage. Rancangan ini adalah mustahil sounds unnatural.

Rancangan can mean plan, scheme, program, or proposal, depending on context.

In this sentence, it most naturally means plan:

• Rancangan ini mustahil = This plan is impossible

In other contexts, rancangan can also refer to a TV or radio program.

In Malay, demonstratives like ini (this) and itu (that) usually come after the noun.

So:

• rancangan ini = this plan
• buku ini = this book
• rumah itu = that house

This is one of the first word-order differences English speakers notice.

Mustahil is an adjective meaning impossible.

In Rancangan ini mustahil, it works like the predicate adjective:

• rancangan ini = the subject
• mustahil = describes the subject

So the structure is basically:

• [Noun phrase] + [adjective]
• Rancangan ini + mustahil

Grammatically, learners may be tempted to do that because English uses is, but in natural Malay, this is usually not the best choice.

For adjective predicates, Malay normally just says:

• Rancangan ini mustahil

Using adalah before an adjective is often considered awkward or too influenced by English. In standard explanations, adalah is more commonly used before noun phrases or in formal definitions, not before adjectives like mustahil.

So the most natural version is:

• Rancangan ini mustahil

Yes. Mustahil is much stronger than sukar or susah.

• mustahil = impossible
• sukar / susah = difficult, hard

So:

• Rancangan ini sukar = This plan is difficult
• Rancangan ini mustahil = This plan is impossible

If you use mustahil, you are saying it cannot realistically be done, not just that it will be hard.

A simple approximate pronunciation is:

• ranch-ah-ngan ee-nee moos-tah-hil

A few notes:

• c in Malay is pronounced like ch in chair
• ng is like the ng in sing
• i is usually pronounced like ee in see
• u is usually like oo in food

So:

• rancangan ≈ rahn-CHAH-ngan
• ini ≈ ee-NEE
• mustahil ≈ moos-tah-HEEL

Malay pronunciation is generally quite regular.

Yes, but the meaning changes slightly.

• Rancangan ini mustahil = This plan is impossible
• Rancangan mustahil = An impossible plan or a plan that is impossible

With ini, you are referring to a specific plan. Without it, the phrase becomes more general or descriptive.

It works in both, but it sounds a little firm and serious because of mustahil.

It is perfectly normal Malay and can be used in speech or writing. In casual conversation, people might also say things like:

• Rancangan ini tak mungkin = This plan is not possible / impossible
• Rancangan ini susah sangat = This plan is very difficult

But Rancangan ini mustahil is still natural and clear.

They are very close, but there can be a slight difference in feel.

• mustahil = impossible
• tak mungkin = not possible / unlikely / impossible, depending on context

Mustahil often sounds more absolute or emphatic.

Compare:

• Rancangan ini mustahil = This plan is impossible
• Rancangan ini tak mungkin = This plan isn’t possible

In many contexts, they can translate the same way, but mustahil may sound a bit stronger or more definite.

Not if you want the same meaning.

• Rancangan ini mustahil = This plan is impossible

But:

• Ini rancangan mustahil would sound like This is an impossible plan, and even then it is not the most natural phrasing.

If you want to say This is an impossible plan, a more natural form would be:

• Ini rancangan yang mustahil
• or more naturally, Ini ialah rancangan yang mustahil in a more formal style

So for your original meaning, keep:

• Rancangan ini mustahil