O gato está sob a mesa.

They are definite articles: o = the for a masculine singular noun, and a = the for a feminine singular noun.

So:

• o gato = the cat
• a mesa = the table

In Portuguese, nouns have grammatical gender, so the article has to match the noun:

• gato is masculine, so it takes o
• mesa is feminine, so it takes a

Portuguese has two main verbs that can both mean to be: ser and estar.

Here, está is used because the sentence talks about location: where the cat is.

• estar is normally used for location and temporary states
• ser is used more for identity, permanent characteristics, time, and definitions

So:

• O gato está sob a mesa. = the cat is under the table
  
  • location → use estar

Using é here would sound wrong in standard Portuguese.

Sob means under or beneath.

It is a correct and standard preposition, but in everyday Brazilian Portuguese, it can sound a bit more formal or written than some other options.

Very often, Brazilians would naturally say:

• debaixo da mesa
• embaixo da mesa

instead of sob a mesa

So sob is absolutely correct, but it may feel slightly more formal or literary in casual speech.

Because sob is followed directly by the article a.

• sob
  • a mesa → sob a mesa

Unlike some other Portuguese prepositions, sob usually does not form a contraction with the article.

For comparison:

• de + a → da
• em + a → na
• por + a → pela

But:

• sob + a stays sob a

So sob a mesa is the expected form.

Not always in a practical sense, but grammatically gato is the masculine form.

• o gato = the male cat / the cat
• a gata = the female cat

In some contexts, the masculine form can be used more generally, especially if the sex of the animal is unknown or not important. But if you specifically know it is female, you would normally say a gata.

The accent mark shows which syllable is stressed.

• está is stressed on the last syllable: es-TÁ

The accent is important because it helps with both pronunciation and spelling.

Without getting too technical, Portuguese uses accent marks to show stress when a word does not follow the most predictable stress pattern. So the accent in está tells you clearly how to say it.

In Brazilian Portuguese, sob is pronounced roughly like sawb or sob, depending on accent and how carefully someone is speaking.

A few notes:

• o here is an open ó sound in many pronunciations
• the final b is pronounced, though at the end of a word it may sound softer than in English

So it is not silent. You should pronounce the b.

It is correct, but in casual speech many Brazilians would more naturally say:

• O gato está debaixo da mesa.
• O gato está embaixo da mesa.

These are very common in conversation.

So you can think of it like this:

• sob a mesa = correct, somewhat more formal or written
• debaixo da mesa / embaixo da mesa = very common in everyday spoken Brazilian Portuguese

Because Portuguese usually follows the same basic word order as English in simple statements:

subject + verb + prepositional phrase

Here:

• O gato = subject
• está = verb
• sob a mesa = prepositional phrase showing location

So the sentence structure is very straightforward and natural in Portuguese.

Normally, no. In standard Portuguese, articles are used much more often than in English, and in this sentence they sound necessary.

So the natural form is:

• O gato está sob a mesa.

Leaving them out would sound unnatural in most normal contexts.

Portuguese very often prefers:

• o / a / os / as

where English might sometimes omit the or use no article at all.