Nasa tabi ng pinto ang susi ko.

Because Filipino often uses predicate-first word order.

In this sentence:

• Nasa tabi ng pinto = the predicate, telling where something is
• ang susi ko = the topic, the thing being talked about

A more literal English-style rendering would be:

• Beside the door is my key

But the natural English translation is:

• My key is beside the door

So the Filipino order is normal, even though it feels reversed to an English speaker.

Nasa is used to say that something is in/on/at a place.

In this sentence, nasa tells you the key is located somewhere:

• Nasa tabi ng pinto = is beside the door

For learning purposes, it is best to treat nasa as a common location word used in sentences about where something is.

Not in the same way as English.

Filipino often has nonverbal sentences, where there is no separate copula like English is/are. Instead, the predicate itself carries the meaning.

So in:

• Nasa tabi ng pinto ang susi ko

the location expression nasa tabi ng pinto functions as the predicate, so you do not need an extra word for is.

It means beside the door or next to the door.

Breakdown:

• sa = at/in/on/to a place
• tabi = side; beside
• ng pinto = of the door

So:

• sa tabi ng pinto literally means at the side of the door
• naturally in English: beside the door

It is very helpful to learn sa tabi ng as a common phrase meaning beside.

Because tabi is followed by the thing whose side you mean, and that noun is introduced by ng.

The pattern is:

• sa tabi ng + noun

Examples:

• sa tabi ng pinto = beside the door
• sa tabi ng mesa = beside the table
• sa tabi ng kotse = beside the car

So sa marks the whole location phrase, while ng links tabi to the noun that follows.

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

Here, ang susi ko is the thing being talked about:

• ang susi ko = my key

It is important not to think of ang as simply meaning English the. Sometimes it overlaps with the, but its real job is grammatical: it marks the noun phrase that is the topic/focus of the clause.

Because short possessive pronouns like ko usually come after the noun they possess.

So:

• susi ko = my key
• bahay ko = my house
• kotse mo = your car

In this sentence:

• ang susi ko = my key

This noun + possessive-pronoun pattern is very common in Filipino.

Yes. That is also correct.

Compare:

• Nasa tabi ng pinto ang susi ko
• Ang susi ko ay nasa tabi ng pinto

They mean the same thing. The difference is mainly structure and style:

• predicate-first: Nasa tabi ng pinto ang susi ko
  
  • very common and natural
• topic-first with ay: Ang susi ko ay nasa tabi ng pinto
  
  • also correct, sometimes a bit more formal or more explicitly structured

So both are fine.

Yes. You could say:

• Katabi ng pinto ang susi ko

This also means My key is beside the door.

The two are very close:

• nasa tabi ng pinto = is at the side of the door
• katabi ng pinto = is beside / next to the door

Both are natural, but katabi is a little more compact.

In modern standard writing, nasa is normally written as one word in this usage.

So write:

• Nasa tabi ng pinto ang susi ko

not:

• Na sa tabi ng pinto ang susi ko

As a learner, the safest approach is to memorize nasa as the standard written form for this common locational expression.

Because Filipino does not use articles the same way English does.

In English, you often need the or a. In Filipino, noun markers like ang, ng, and sa do not match English articles exactly.

So:

• ng pinto can mean something like of the door or of a door, depending on context

In this sentence, it will usually be understood as the door from the situation or previous context.