Isso é impossível.

Isso is a demonstrative pronoun. It usually points to:

• something just mentioned
• an idea or situation
• something you can indicate but do not name directly

In Brazilian Portuguese, isso is very common when reacting to what someone just said:

• Isso é verdade.
• Isso faz sentido.
• Isso é impossível.

A useful shortcut:

• isto = this
• isso = that / this thing / that idea
• aquilo = that over there / that thing

In everyday Brazilian Portuguese, isso is much more common than isto in many situations.

Because é comes from ser, and ser is used for things like:

• identity
• definition
• classification
• general judgments

Here, the speaker is judging something as impossible, so ser is the normal choice:

• Isso é impossível.

If you use está, the meaning often becomes more temporary or situation-based:

• Está impossível. = It’s impossible right now / It’s unbearable right now.
• O trânsito está impossível hoje. = The traffic is impossible today.

So in your sentence, é is the most natural and standard choice.

A simple English-friendly approximation is:

EE-soo eh eem-poh-SEE-vew

A few key points:

• isso → EE-soo
• é → eh (like the e in bet, but more open)
• impossível → stress on sí: eem-poh-SEE-vew

More pronunciation notes:

• ss in isso and impossível sounds like a normal English s
• the im- before p is slightly nasal in Portuguese
• in most of Brazil, final l sounds like a soft w, so -vel sounds close to vew

The accent in é is important.

It helps distinguish:

• é = is
• e = and

So:

• Isso é impossível. = That is impossible.
• Isso e impossível. = incorrect

The accent also shows the vowel pronunciation: é is stressed and pronounced clearly.

The accent shows where the stress goes.

Without the accent, a learner might guess the stress incorrectly.
With the accent, you know the stressed syllable is:

im-pos-SÍ-vel

That is why the word is written impossível, not impossivel.

In standard Portuguese writing, accents are not optional. They are part of the correct spelling.

It is a very close cognate, so it is easy to recognize. But notice a few differences:

• English: impossible
• Portuguese: impossível

Important differences:

• Portuguese uses -ível
• the stress is different
• the pronunciation is different
• the spelling must include the accent

So yes, the words are closely related, but you should learn the Portuguese form as its own word.

Yes. Isso não é possível is completely natural.

There is a small nuance:

• Isso é impossível. = stronger, more direct
• Isso não é possível. = not possible; sometimes slightly less absolute

In real speech, both are common.

Also, Não é possível! can be an exclamation meaning something like:

• No way!
• That can’t be!
• I can’t believe it!

So the two forms overlap, but impossível often sounds more emphatic.

Yes — very often.

That is one of the main jobs of isso. It can refer to:

• a statement
• a plan
• a situation
• something someone did
• an entire previous sentence

Examples:

• Você quer terminar tudo hoje? Isso é impossível.
• Ele disse que vai sozinho. Isso é perigoso.

So in your sentence, isso does not have to mean a concrete object. It can mean that idea or what you just said.

You can, but it sounds more marked and less neutral.

The most normal, straightforward order is:

• Isso é impossível.

If you say:

• É impossível isso.

it can sound more emotional, emphatic, or conversational, almost like:

• That’s impossible, that.
• Impossible, that is.

So learners should usually stick with Isso é impossível first.

Yes, absolutely.

Portuguese often drops the rest of the sentence when the meaning is obvious from context.

So all of these are possible:

• Isso é impossível.
• Não é possível.
• Impossível!

By itself, Impossível! is a very natural reaction in conversation. It is short, strong, and expressive.