senzyuuneko ha sizuka desu.

It is commonly read せんじゅうねこ.

• 先住 = already living there / pre-existing / resident
• 猫 = cat

So 先住猫 refers to the cat that was already in the home before a new cat arrived.

This is a useful pet-related term, but it is not one of the very first everyday vocabulary words beginners usually learn.

は marks the topic of the sentence.

In 先住猫 は 静か です, the topic is 先住猫.
The sentence is saying something about that cat: it is quiet.

A natural way to think about it is:

• As for the resident cat, it is quiet.

This does not always translate directly into natural English, but it helps explain the grammar.

Because 静か is not an い-adjective. It is a な-adjective.

Japanese has two main adjective types:

• い-adjectives: like 高い, 大きい
• な-adjectives: like 静か, きれい, 便利

So you do not say 静かい.

When a な-adjective comes at the end of a sentence, it is followed by です:

• 静かです = is quiet

Because な is only used when the adjective comes directly before a noun.

Compare:

• 静かな猫 = a quiet cat
• 猫は静かです = the cat is quiet

In your sentence, 静か is not modifying a noun directly. It is the predicate of the sentence, so you use です, not な.

Japanese normally does not use articles like English a or the.

So 先住猫 can mean things like:

• the resident cat
• a resident cat
• our resident cat

Which one feels right depends on context.

In real conversation, the listener usually already knows which cat is being talked about.

Sometimes yes, but the nuance changes.

• 先住猫は静かです。
  = As for the resident cat, it is quiet.
  This is a neutral topic-comment statement.

• 先住猫が静かです。
  = It is the resident cat that is quiet / the resident cat is quiet.
  This can sound more specific, contrastive, or like answering a question such as Which cat is quiet?

For a simple general statement, は is the more natural choice.

Yes, it is grammatical and understandable.

However, by itself it can sound a little context-dependent, because 先住猫 is a somewhat specific label. In actual conversation, people might often say something slightly fuller, such as:

• うちの先住猫は静かです。 = Our resident cat is quiet.
• 先住の猫は静かです。 = The cat that was already here is quiet.

So your sentence is fine, but in real life it often appears with a little more context.

Yes. 先住の猫 is also possible and may feel a bit more transparent to learners, because it clearly shows:

• 先住 = already living there
• の = linking phrase
• 猫 = cat

So:

• 先住猫 = resident cat / pre-existing cat
• 先住の猫 = the cat that was already living there

Both are understandable. The compound 先住猫 is common in pet-adoption or multi-cat contexts.

It is non-past polite.

So 静かです can mean:

• is quiet
• quiet (in a polite statement)

If you want past tense:

• 先住猫は静かでした。 = The resident cat was quiet.

If you want negative:

• 先住猫は静かではありません。 = The resident cat is not quiet.

Grammatically, it is marked as the topic because it has は.

In beginner explanations, people sometimes loosely call it the subject in English because it is the main thing the sentence is about. But more precisely:

• 先住猫 = topic
• 静かです = comment/predicate about that topic

This topic-comment structure is very common in Japanese.

The spaces are usually just for learners.

Normal Japanese writing would usually be:

先住猫は静かです。

Japanese does not normally put spaces between words the way English does. Textbooks and study materials often add spaces to make the structure easier to see.