Nabubulok ang kalat.

Filipino often puts the predicate first. In Nabubulok ang kalat, the predicate is nabubulok and the topic marked by ang is kalat.

So the natural Filipino order is:

Nabubulok ang kalat.
literally something like Is rotting the trash.

In smoother English, of course, we say The trash is rotting.

You can also say:

Ang kalat ay nabubulok.

That is also correct, but it is a more explicitly topic-first version.

Nabubulok is a verb form built from the root bulok, which has the idea of rot, decay, or being rotten.

A useful way to think of it is:

• bulok = rotten / rot
• mabulok = to rot, to become rotten
• nabubulok = is rotting / is in the process of rotting

So in this sentence, nabubulok describes an ongoing process.

The root word is bulok.

Bulok by itself is often used as an adjective meaning rotten.
When turned into a verb form such as mabulok or nabubulok, it expresses the process of becoming rotten or decaying.

Ang is a marker that highlights the topic of the sentence.

In Nabubulok ang kalat, ang kalat is the thing being talked about: the trash, litter, or mess.

English learners often want to translate ang directly as the, and that sometimes works, but it is better to think of ang mainly as a topic marker, not a perfect equivalent of English the.

Not exactly in a one-to-one grammatical sense.

In this sentence, ang kalat is naturally translated as the trash or the litter, because that sounds normal in English. But ang is not simply the Filipino word for the.

It marks the noun phrase as the topic or focused element of the sentence. Depending on context, ang kalat could be understood as:

• the trash
• the mess
• the litter
• sometimes even just the scattered stuff

So the best English translation depends on context.

Kalat can mean several related things:

• mess
• clutter
• litter
• scattered things
• sometimes trash, depending on context

So yes, it can be translated as trash here, but the word is a little broader than basura.

If you want a more specific word for garbage/trash, basura is often more direct.
Kalat can suggest that things are strewn around or making a mess.

A simple distinction is:

• basura = garbage, refuse, trash
• kalat = mess, litter, scattered clutter

So:

• basura focuses on the fact that something is waste
• kalat focuses on the fact that something is scattered around or untidy

If you are talking about decomposing garbage, basura may sound more straightforward.
If you are talking about litter or a messy pile of trash, kalat can work well too.

It is better to talk about aspect rather than tense in Filipino.

Nabubulok expresses an ongoing or incomplete action/state, similar to English:

• is rotting
• is decaying

So instead of thinking present tense, think ongoing aspect.

This is an important difference:

• Nabubulok ang kalat = the trash is rotting / is in the process of decaying
• Bulok ang kalat = the trash is rotten

So:

• nabubulok emphasizes the process
• bulok describes the condition

English has a similar contrast:

• is rotting = process
• is rotten = state

Yes. That is grammatical.

Compare:

• Nabubulok ang kalat.
• Ang kalat ay nabubulok.

Both mean essentially the same thing. The first is the more typical predicate-first structure. The second puts ang kalat first and uses ay, which can sound more formal, more careful, or more contrastive depending on context.

It can function a bit like a mass noun or a collective idea.

So ang kalat can refer to:

• a mess in general
• scattered litter
• trash as a whole

You do not always need mga. If you want to emphasize multiple separate items, you could say ang mga kalat, but in many everyday contexts, kalat by itself already works naturally.

It is better understood as a change of state happening to the trash.

The trash is not actively doing something to another object. Instead, it is undergoing decay.

So in English, is rotting is a good translation because it also works that way: the subject is the thing experiencing the process.

A simple pronunciation guide is:

na-bu-bu-lok

The stress is on the last syllable: na-bu-bu-LOK.

And kalat is:

ka-lat

with stress usually on the second syllable: ka-LAT.

So the whole sentence sounds roughly like:

na-bu-bu-LOK ang ka-LAT

Yes, it is understandable and grammatical.

That said, in everyday speech, many speakers might choose basura if they specifically mean garbage:

Nabubulok ang basura.

That can sound more direct for trash/garbage.
Still, Nabubulok ang kalat is natural if the intended meaning is something like the litter/mess is rotting.