Isso é perigoso.

Isso is a demonstrative pronoun. In this sentence, it points to that idea, thing, action, or situation.

In Brazilian Portuguese, isso is very commonly used when:

• referring to something just mentioned
• reacting to what someone is doing
• pointing to something conceptually close to the listener or to the conversation

For example, if someone says they want to climb a broken ladder, you could say Isso é perigoso to mean that what they are talking about is dangerous.

Because é comes from ser, which is usually used for:

• general characteristics
• definitions
• classifications
• things seen as inherently or basically true

So Isso é perigoso means something like That is dangerous in a general or essential sense.

If you said Isso está perigoso, using estar, it would usually mean:

• it is dangerous right now
• it has become dangerous
• the current situation is dangerous

So the difference is often:

• Isso é perigoso = that is dangerous by nature / in general
• Isso está perigoso = that is dangerous at the moment / has gotten dangerous

Because isso takes masculine singular agreement.

Even though isso does not refer to a male person, Portuguese normally uses the masculine singular form of the adjective after pronouns like isso, isto, and aquilo.

So:

• Isso é perigoso
• Aquilo é estranho
• Isto é bom

If you used a feminine noun instead, the adjective would change:

• Essa situação é perigosa
• Essa ideia é perigosa

Yes, grammatically you can.

Traditionally:

• isto = this
• isso = that
• aquilo = that over there / that yonder

But in everyday Brazilian Portuguese, isto is much less common than isso. Many speakers use isso in situations where a textbook might suggest isto.

So isto é perigoso is correct, but isso é perigoso sounds more natural in most spoken Brazilian Portuguese.

Yes — very often.

That is one of the most useful things about isso. It can refer to:

• a physical thing
• an idea
• something someone just said
• an action
• an entire situation

For example:

• Someone says: Vou dirigir sem cinto.
  You reply: Isso é perigoso.

Here, isso does not mean one object. It refers to the whole action of driving without a seat belt.

A simple Brazilian Portuguese approximation is:

EE-so eh peh-ree-GOH-zoo

A slightly more accurate pronunciation is:

/ˈisu ɛ peɾiˈgozu/

A few helpful notes:

• isso sounds like EE-soo, with stress on the first syllable
• é sounds like eh
• in Brazilian Portuguese, the final -o often sounds like -u
• the r in perigoso is a light tap, somewhat like the quick tt in American English butter

The accent in é is important.

It shows that:

1. the word is stressed
2. the vowel is an open e sound
3. it is different from e, which means and

So:

• é = is
• e = and

This is a very common distinction in Portuguese, so the accent should always be written.

Yes, but the meaning becomes less specific.

• Isso é perigoso = That is dangerous
• É perigoso = It is dangerous / That’s dangerous / It’s dangerous to do that, depending on context

So if the context already makes it obvious what you are talking about, É perigoso is possible. But if you want to clearly point to a specific thing or idea, isso is useful.

It is neutral and natural in both speech and writing.

Isso é perigoso works well in:

• casual conversation
• warnings
• classroom language
• everyday writing

It is not especially formal or especially informal. It is just a standard, normal sentence.

The adjective must agree with the noun it describes.

With isso, you use masculine singular:

• Isso é perigoso

But with other nouns:

• Essa situação é perigosa
• Esse lugar é perigoso
• Essas substâncias são perigosas
• Esses animais são perigosos

So the endings change:

• perigoso = masculine singular
• perigosa = feminine singular
• perigosos = masculine plural
• perigosas = feminine plural

No. It can describe several kinds of danger.

It can mean:

• physically dangerous
• risky
• harmful
• threatening
• dangerous in a figurative sense

For example:

• É perigoso dirigir cansado. = physically risky
• Isso é uma ideia perigosa. = potentially harmful idea
• Ele é um homem perigoso. = dangerous person

So in Isso é perigoso, the danger could be literal or figurative, depending on context.