Nakasara ang pinto dahil malamig ngayong gabi.

In Filipino, the predicate often comes first. So Nakasara ang pinto is a very normal structure.

• Nakasara = the predicate, describing the state
• ang pinto = the topic/subject-like noun phrase

A more English-like order is also possible:

• Ang pinto ay nakasara.

Both mean the same thing, but predicate-first word order is very common in everyday Filipino.

Nakasara means closed or shut, specifically in a closed state.

It comes from the root sara and the prefix naka-, which often shows a state or condition resulting from an action.

So:

• nakasara = closed / in a shut position
• not closing
• not to close

It describes the condition of the door, not the action of someone closing it.

Both nakasara and sarado can mean closed, and in many situations they are interchangeable.

A rough distinction:

• sarado = simply closed
• nakasara = in a closed state, often with a slightly more result-oriented feel

So both of these are natural:

• Sarado ang pinto.
• Nakasara ang pinto.

For many learners, it is enough to know that both are common and correct, though nakasara feels a bit more like a built Filipino form, while sarado is a very common borrowed adjective.

Ang is a marker for the topic of the sentence. In many simple sentences like this one, it often lines up with what English speakers think of as the subject.

So in:

• Nakasara ang pinto

ang pinto is the thing being talked about: the door.

A very useful beginner understanding is:

• ang often marks the main noun phrase being described

There is no separate word for is here, because Filipino often does not use a copula in sentences like this.

So:

• Nakasara ang pinto literally looks like Closed the door
• but naturally means The door is closed

Likewise:

• Malamig ngayong gabi = It is cold tonight

This is one of the big differences from English: Filipino often expresses description without a verb like is/am/are.

Because dahil by itself can introduce a clause, while dahil sa usually comes before a noun phrase.

Here, what follows is a clause-like idea:

• dahil malamig ngayong gabi
• because it is cold tonight

But if you wanted because of + noun, you would use dahil sa:

• dahil sa lamig = because of the cold
• dahil sa panahon = because of the weather

So:

• dahil + clause
• dahil sa + noun phrase

In English, weather expressions usually need a dummy subject:

• It is cold
• It is raining

In Filipino, that dummy it is usually not needed.

So:

• Malamig. = It is cold.
• Umuulan. = It is raining.

That is why dahil malamig ngayong gabi is complete and natural even without a separate word for it.

Ngayong is ngayon plus the linker -ng.

• ngayon = now
• ngayong gabi = tonight / this evening / this night

The -ng links ngayon to the following word gabi.

This kind of linking is very common in Filipino:

• magandang umaga
• malamig na gabi
• ngayong gabi

So ngayong gabi is the natural form, not ngayon gabi.

Yes. You can say:

• Dahil malamig ngayong gabi, nakasara ang pinto.

This is also correct and natural. It just changes the emphasis slightly:

• Nakasara ang pinto dahil malamig ngayong gabi.
  Focus first on the door being closed
• Dahil malamig ngayong gabi, nakasara ang pinto.
  Focus first on the reason

Both are good Filipino.