Daun itu rosak bukan kerana hujan, tetapi kerana ulat.

A natural breakdown is:

• daun = leaf / leaves
• itu = that / those
• rosak = damaged, spoiled, ruined
• bukan = not
• kerana = because of / because
• hujan = rain
• tetapi = but
• ulat = caterpillar / worm

So the structure is basically:

• Daun itu rosak = The leaf is damaged / Those leaves are damaged
• bukan kerana hujan = not because of the rain
• tetapi kerana ulat = but because of caterpillars / worms

In Malay, demonstratives like ini and itu usually come after the noun.

So:

• daun itu = that leaf / those leaves
• rumah itu = that house
• orang itu = that person

This is different from English, where we say that leaf with that before the noun.

Malay often does not use a copula like English is/are in simple descriptions.

So:

• Daun itu rosak literally looks like that leaf damaged
• but it means That leaf is damaged

This is very normal in Malay. The adjective or describing word can come directly after the noun.

Other examples:

• Rumah itu besar = That house is big
• Buku ini baharu = This book is new

This is a very common learner question.

A simple rule is:

• tidak usually negates verbs and adjectives
• bukan usually negates nouns and is also used in certain contrast structures

In this sentence, bukan kerana ..., tetapi kerana ... is a fixed and very common pattern meaning:

• not because of X, but because of Y

So bukan is the natural choice here.

Compare:

• Daun itu tidak rosak = The leaf is not damaged
• Daun itu rosak bukan kerana hujan = The leaf is damaged not because of rain

It is a contrast pattern meaning:

• not because of X, but because of Y

It corrects or rejects one explanation and gives the real one.

So here:

• bukan kerana hujan = not because of rain
• tetapi kerana ulat = but because of caterpillars / worms

You can use the same pattern in many sentences:

• Dia lewat bukan kerana malas, tetapi kerana keretanya rosak.
  He was late not because he was lazy, but because his car broke down.

Both can often mean because.

• kerana is usually a bit more formal or neutral in writing
• sebab is very common in everyday speech

So this sentence could also be said as:

• Daun itu rosak bukan sebab hujan, tetapi sebab ulat.

That sounds more conversational.
The version with kerana sounds a bit more standard or polished.

Yes. Malay nouns usually do not have to change form for singular vs plural.

So daun can mean:

• leaf
• leaves

And daun itu can mean:

• that leaf
• those leaves

The exact meaning depends on context.

If you really want to make it clearly plural, Malay can use reduplication:

• daun-daun itu = those leaves

But in many real sentences, Malay leaves number unstated unless it matters.

Malay does not use articles like English a/an/the in the same way.

So:

• hujan can mean rain or the rain, depending on context
• ulat can mean a caterpillar, caterpillars, worms, etc.

English forces you to choose more precisely. Malay often leaves that information implicit unless it is important.

Literally, ulat can refer to a worm-like creature, and in many everyday plant contexts it often means caterpillar.

In this sentence about leaves being damaged, an English speaker would often understand it as:

• caterpillars
• or more loosely worms/pests

The exact translation depends on context, but caterpillars is often the most natural choice when talking about leaf damage.

Yes. Malay allows some flexibility.

For example:

• Daun itu rosak kerana ulat, bukan kerana hujan.

This still means essentially the same thing:

• The leaf was damaged because of caterpillars, not because of rain.

The original version puts stronger focus on correcting the wrong cause first:

• not because of rain, but because of caterpillars

So the difference is mostly about emphasis, not basic meaning.

No. Tetapi is standard and very common in writing.

You may also see:

• tapi = a shorter, more informal spoken form

So in casual speech, people might say:

• Daun itu rosak bukan kerana hujan, tapi kerana ulat.

Both are correct; tetapi is just more formal.

In everyday speech, speakers might say a slightly more casual version such as:

• Daun itu rosak bukan sebab hujan, tapi sebab ulat.

This is less formal but very natural conversational Malay.

The original sentence is fully correct and natural too; it just sounds a bit more standard or written because of kerana and tetapi.