Keputusan itu mempunyai kesan besar.

A natural word-by-word breakdown is:

• keputusan = decision or result, depending on context
• itu = that (and sometimes it also makes the noun feel like the)
• mempunyai = to have
• kesan = effect or impact
• besar = big, great, or major

So the structure is:

• Keputusan itu = that decision / the decision
• mempunyai kesan besar = has a big effect / has a major impact

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

So:

• keputusan itu = that decision
• rumah itu = that house
• orang ini = this person

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

Malay adjectives usually come after the noun they describe.

So:

• kesan besar = big effect
• rumah besar = big house
• masalah kecil = small problem

This noun + adjective order is normal in Malay.

Mempunyai means to have or to possess.

It is correct and natural, but it sounds a bit more formal or written than some other options. In everyday speech, many speakers would also say:

• Keputusan itu ada kesan besar.

That is more conversational.
Meanwhile:

• Keputusan itu mempunyai kesan besar.

sounds a bit more formal, careful, or written.

Another common formal alternative is:

• Keputusan itu memberi kesan besar.
  = The decision has/makes a big impact.

No. Keputusan can mean different things depending on context, especially:

• decision
• result
• outcome
• sometimes verdict

For example:

• keputusan mahkamah = court decision / verdict
• keputusan peperiksaan = exam results

So in this sentence, decision is likely the intended meaning, but the word itself is broader than that.

Malay does not normally use articles like a, an, and the the way English does.

So kesan besar can mean:

• a big effect
• big impact
• major effect

The exact English choice depends on context.

Malay often leaves article-like meanings unstated unless something specific is needed. In this sentence, itu helps make keputusan specific, but kesan besar does not need an article word.

No. Kesan besar is already correct.

In simple noun + adjective combinations, yang is often not needed:

• kesan besar = big effect
• rumah besar = big house

You may also see:

• kesan yang besar

This is also grammatical, but it sounds a bit more explicit or slightly more formal in some contexts. For a learner, the safest basic pattern is:

• noun + adjective
  so here: kesan besar

No. Here it is figurative.

Besar literally means big, but it is often used for degree or importance too. So kesan besar means:

• big effect
• great effect
• major impact
• significant impact

It does not mean the effect is physically large.

Yes, both are derived forms, which is very common in Malay.

keputusan

This comes from the root putus, which relates to ideas like decide or be resolved/cut off, with the circumfix ke-...-an forming a noun.

So keputusan is a noun meaning something like:

• decision
• result
• outcome

mempunyai

This comes from punya. With the mem- verbal prefix, it becomes mempunyai, meaning:

• to have
• to possess

You do not need to analyze these every time you read them, but knowing Malay often builds words through affixes can help a lot.

A very common casual version is:

• Keputusan itu ada kesan besar.

This is simpler and very natural in speech.

Other natural alternatives include:

• Keputusan itu memberi kesan besar.
• Keputusan itu membawa kesan besar.

These all express a similar idea, but with slightly different style:

• mempunyai = more formal, has
• ada = casual everyday has
• memberi = gives / has an impact
• membawa = brings

You can negate the verb with tidak:

• Keputusan itu tidak mempunyai kesan besar.

That means:

• The decision does not have a big effect.
• The decision does not have a major impact.

For verbs like mempunyai, use tidak, not bukan.

So:

• tidak mempunyai = correct
• bukan mempunyai = not correct in normal usage