teeburu ha anteisite inai.

は marks テーブル as the topic of the sentence.

So テーブルは安定していない is literally something like:

• As for the table, it is not stable.

In natural English, we usually just say:

• The table isn’t stable.
• The table is unstable.
• The table is wobbly.
  (depending on the situation)

The important point is that は does not directly mean is. It tells you what the sentence is about.

テーブル is a loanword borrowed from English table, so it is written in katakana.

Katakana is commonly used for:

• foreign loanwords
• some onomatopoeia
• emphasis
• scientific names, etc.

So:

• テーブル = table
• 机 = desk/table, but usually a more native Japanese word

In this sentence, テーブル sounds natural if you mean an actual table, especially a Western-style one.

This is a very common learner question.

安定 is basically a noun / する-noun meaning stability or being stable.
Because of that, Japanese often uses:

• 安定する = to become stable / to be stable
• 安定している = is stable
• 安定していない = is not stable

This is the most natural way to describe the table’s condition here.

You might also see or hear:

• 安定ではない
• 安定じゃない

But those sound more abstract or less natural in this kind of physical description. For a wobbly table, 安定していない is much more natural.

Not always.

In many cases, 〜ている can mean an ongoing action:

• 食べている = is eating

But with some verbs, especially ones about a resulting state or condition, 〜ている describes the current state:

• 結婚している = is married
• 知っている = knows
• 安定している = is stable

So here:

• 安定していない does not mean the table is actively not stabilizing
• it means the table is in an unstable state

That is why the natural translation is is not stable.

Not exactly in the same way as 高い or 静か.

It is best to think of 安定 as a noun-like word that often combines with する:

• 安定する = to stabilize / to be stable
• 安定している = to be in a stable condition

Compare:

• 静かだ = is quiet
• 安定している = is stable

So even though English uses a simple adjective stable, Japanese often uses this noun + する + ている pattern instead.

Normally, standard Japanese is written without spaces.

So the usual writing would be:

• テーブルは安定していない。

The spaces in your sentence are probably there only to help learners see the parts more clearly.

Because the sentence is built from 安定している.

Let’s break it down:

• 安定する → dictionary form
• 安定している → is stable
• 安定していない → is not stable

So the negative is made by negating いる:

• いる → いない

This is the normal way to negate a 〜ている form.

Here it means the current state:

• The table is not stable.

It is not usually understood as:

• The table is not becoming stable.

That is because 安定している / 安定していない is commonly used to describe a condition.

So in this sentence, the focus is on how the table is now.

Yes, and it would be grammatical.

The difference is mainly one of focus:

• テーブルは安定していない
  = As for the table, it isn’t stable.
  This presents the table as the topic.

• テーブルが安定していない
  = The table is the one that isn’t stable.
  This can sound more like you are identifying or emphasizing the table as the thing with the problem.

In many everyday situations, either could work, but は is very natural for a simple statement about the table.

It is not better, but it is more polite.

Compare:

• 安定していない。 = plain style
• 安定していません。 = polite style

You would choose based on the situation:

• plain style: casual conversation, notes, informal writing
• polite style: speaking politely, customer service, formal situations

Both mean the same basic thing.

Yes, very often that is exactly the idea.

For a physical object like a table, 安定していない often suggests:

• it wobbles
• it is shaky
• it does not stand firmly
• it is not steady

So while the direct meaning is not stable, the practical meaning in context may be something like:

• The table is wobbly.

Yes. In casual speech, していない is often shortened to:

• してない

So you may hear:

• テーブルは安定してない。

That is very common in conversation.
But for careful writing and standard textbook form, していない is the full form.

The standard written form is:

• テーブルは安定していない。

A polite version would be:

• テーブルは安定していません。

A casual spoken version might be:

• テーブル、安定してない。

In speech, Japanese often drops particles or softens things depending on context, but the textbook version with は is a good model to learn.