O cachorro está em perigo.

O is the masculine singular definite article, meaning the.

• o cachorro = the dog
• a casa = the house (feminine)

In Portuguese, nouns usually need an article more often than in English, especially in simple statements like this.

Cachorro is the masculine form of dog. Cachorra is the feminine form.

So:

• o cachorro = the male dog or sometimes just the dog
• a cachorra = the female dog

In many everyday situations, Portuguese uses the masculine form as the general/default word when the sex of the animal is unknown or unimportant.

Está is the he/she/it form of the verb estar, which often means to be in the sense of a temporary state, condition, or location.

Here it means is:

• O cachorro está em perigo. = The dog is in danger.

Portuguese has two main verbs for to be: ser and estar. In this sentence, estar is used because being in danger is treated as a current condition or state.

Portuguese distinguishes between two kinds of to be:

• ser → for permanent traits, identity, origin, time, etc.
• estar → for states, conditions, and many locations

Being in danger is considered a condition, not a permanent identity. So Portuguese uses estar:

• O cachorro está em perigo. = The dog is in danger.

Using é here would sound wrong in normal Portuguese.

The accent in está shows the stressed syllable and is part of the correct spelling.

• está is pronounced with stress on the second syllable: es-TÁ

This accent is important in writing. Portuguese uses written accents much more than English does.

Em perigo literally means in danger.

• em = in
• perigo = danger

This is a common expression in Portuguese, just like in English:

• estar em perigo = to be in danger

So the sentence is built very naturally:

• O cachorro = the dog
• está = is
• em perigo = in danger

Yes, but it means something different.

• O cachorro está em perigo = The dog is in danger
• O cachorro está perigoso = The dog is dangerous / The dog is acting dangerously

This is a very important difference:

• em perigo describes the dog's situation
• perigoso describes the dog's quality or behavior

So if you want to say the dog is the one at risk, use em perigo.

Yes, in this sentence the word order is very similar to English:

• O cachorro = subject
• está = verb
• em perigo = prepositional phrase

So it follows a normal pattern:

Subject + verb + expression

That makes this sentence fairly easy for an English speaker to understand.

Normally, no—not in standard Portuguese for this kind of sentence.

The most natural version is:

• O cachorro está em perigo.

Without the article, it sounds incomplete or unnatural in most ordinary contexts. Portuguese often uses articles where English may not focus on them as much.

A simple approximate pronunciation for an English speaker is:

oo kah-SHO-hoo es-TA em peh-REE-goo

A few notes:

• cachorro in Brazilian Portuguese usually sounds like ca-SHO-rro, though the exact rr sound varies by region
• está = es-TA
• perigo = pe-RI-go

A more natural Brazilian rhythm would connect words smoothly, something like:

u ka-SHO-hu es-TA eyn pe-REE-gu

Pronunciation varies by region, but the stress pattern is the key thing to notice.

No. Another common word is cão.

• cachorro = dog, very common in everyday Brazilian Portuguese
• cão = dog, also correct, but often feels more formal, dictionary-like, or used in certain expressions

For most everyday Brazilian speech, cachorro is the most common choice.

Usually O cachorro suggests a specific dog: the dog.

For example, if you are already talking about a certain dog, this fits perfectly:

• O cachorro está em perigo. = The dog is in danger.

If you wanted to say a dog is in danger, you would usually say:

• Um cachorro está em perigo.

So the article helps show whether the dog is specific or not.

Yes. Some common alternatives are:

• em risco = at risk
• correndo perigo = in danger / facing danger
• ameaçado = threatened

Examples:

• O cachorro está em risco.
• O cachorro está correndo perigo.
• O cachorro está ameaçado.

All are possible, but está em perigo is a very direct and common way to say it.