Malaki ang apartment.

Because Filipino very often uses predicate-first word order.

In this sentence, malaki is the predicate, and ang apartment is the topic/subject. So the structure is:

Malaki + ang apartment
literally, something like Big the apartment

That sounds unusual in English, but it is normal in Filipino.

Ang is a marker that tells you which noun is the topic or subject-like part of the sentence.

So in Malaki ang apartment, ang apartment is the thing being talked about.

It is sometimes translated like the, but that is not its full job. It is better to think of ang as a grammatical marker, not a direct one-word equivalent of English the.

There is no separate word for is in this kind of present-tense Filipino sentence.

So:

• Malaki ang apartment = The apartment is big
• not Malaki is ang apartment

This is completely normal. Filipino often leaves out the present-tense copula is/are in sentences like this.

Yes. Ang apartment ay malaki is also correct.

Both mean the same basic thing:

• Malaki ang apartment
• Ang apartment ay malaki

The first one is very common and natural in everyday speech.
The ay version can sound a bit more formal, written, or carefully structured.

Good question. The difference is whether the adjective is the predicate or is directly modifying the noun.

• Malaki ang apartment = The apartment is big
  Here, malaki is the predicate.

• Malaking apartment = big apartment
  Here, malaki directly modifies apartment, so it takes the linker -ng and becomes malaking.

So:

• malaki = predicate form here
• malaking = adjective directly attached to a noun

Yes, apartment is commonly used. Filipino borrows a lot of vocabulary from English, especially for modern everyday things.

You may also encounter other forms depending on speaker, region, or style, but apartment is perfectly natural and widely understood.

It does not change for gender, because Filipino does not work like languages such as Spanish or French, where adjectives agree with masculine/feminine nouns.

For plural, Filipino usually marks the noun with mga, and the adjective may also appear in a plural/distributive form:

• Malaki ang apartment = The apartment is big
• Malalaki ang mga apartment = The apartments are big

So there is no gender agreement, and number works differently from English.

No, not here.

Si is used for personal names or specific people, not for common nouns like apartment.

So:

• ang apartment = correct
• si apartment = incorrect

Use ang for common nouns in this kind of sentence.

Not necessarily. In Filipino, putting the predicate first is often just the normal sentence pattern, not special emphasis.

So Malaki ang apartment usually just means The apartment is big, in a normal way.

If you want stronger emphasis, Filipino usually shows that through context, intonation, or extra words, not just because malaki comes first.

A simple approximation is:

• ang ≈ ahng
  The ng sounds like the end of song

• malaki ≈ mah-lah-KEE
  with the stress on the last syllable

That is a rough English-friendly guide, but it will get you close enough as a beginner.