Tambahan data itu sah.

Not exactly.

• data tambahan usually means additional/extra data
• tambahan data more literally means an addition of data, a data supplement, or the added data as a noun phrase

So word order changes the default meaning. If you specifically want the additional data is valid, Data tambahan itu sah is usually clearer.

Tambahan comes from the root tambah (to add) plus the suffix -an.

That often turns it into a noun meaning things like:

• addition
• supplement
• extra amount/item

In this sentence, tambahan is probably the main noun in the phrase tambahan data.

Itu is a demonstrative, usually meaning that, but it can also make a phrase feel definite, a bit like the in English.

In Malay, demonstratives normally come after the whole noun phrase:

• buku itu = that book / the book
• rumah besar itu = that big house
• tambahan data itu = that data addition / the data addition

So its position is normal in Malay, even though it feels reversed from English.

Because Malay often leaves out the copula (is/am/are) in simple sentences.

So:

• Tambahan data itu sah.

naturally means:

• That data addition is valid.

The adjective sah can directly follow the subject. Malay does this very often.

Yes, you can. It sounds more formal or written.

• Tambahan data itu sah = natural, common
• Tambahan data itu adalah sah = more formal/emphatic

In everyday Malay, leaving out adalah is very normal.

Sah is an adjective meaning things like:

• valid
• legitimate
• officially accepted
• legal (depending on context)

It is often used for documents, procedures, approvals, marriages, votes, registrations, and similar formal matters.

Usually no.

Sah is more about validity or legitimacy, not factual truth.

Compare:

• sah = valid / legally or officially accepted
• benar or betul = true / correct

So a statement can be benar, but a document can be sah.

Use tidak:

• Tambahan data itu tidak sah. = That data addition is not valid.

For adjectives like sah, tidak is the normal negator.

The structure is:

• Tambahan data itu = subject noun phrase
• sah = predicate adjective

So the pattern is basically:

• [noun phrase] + [adjective]

This is a very common Malay sentence pattern.

Not by itself. Malay nouns usually do not change form for singular vs. plural.

So data can refer to:

• data in general
• one set of data
• multiple pieces/sets of data

The context usually tells you which is meant.