A chave está embaixo da mesa.

Because Portuguese usually uses estar for location.

• A chave está embaixo da mesa. = The key is under the table.
• estar is used for where something is.
• ser (as in é) is used more for identity, definition, or permanent characteristics.

So A chave é embaixo da mesa sounds wrong in normal Portuguese.

Embaixo means under, underneath, or below.

In this sentence, embaixo da mesa means under the table.

A very similar word is debaixo, which also means under/beneath. In Brazilian Portuguese, both are common:

• A chave está embaixo da mesa.
• A chave está debaixo da mesa.

Both are natural.

Because de + a contracts to da in Portuguese.

So:

• de + a = da
• de + o = do
• de + as = das
• de + os = dos

In this sentence, embaixo da mesa literally contains the idea of under of the table, but in natural Portuguese that becomes embaixo da mesa.

A here is the feminine singular definite article, meaning the.

• a chave = the key

Portuguese nouns usually need articles more often than English. Since chave is a feminine noun, the article is a, not o.

For the same reason: mesa is also a feminine singular noun.

• a mesa = the table

Because the sentence refers to a specific table, Portuguese uses the definite article:

• da mesa = of the table / the table after contraction

In Portuguese, nouns have grammatical gender. Both chave and mesa are feminine, so they take:

• a in the singular
• as in the plural

Examples:

• a chave
• as chaves
• a mesa
• as mesas

There is not always a logical reason, so gender often has to be learned with the noun.

In modern Brazilian Portuguese, embaixo is normally written as one word.

So write:

• embaixo

not:

• em baixo

In this sentence, the correct spelling is A chave está embaixo da mesa.

The word order is very close to English:

• A chave = the subject
• está = the verb
• embaixo da mesa = location phrase

So the pattern is:

Subject + verb + place

That is why it feels familiar to English speakers:

• The key is under the table.
• A chave está embaixo da mesa.

Usually, no. That sounds unnatural in normal Portuguese.

Portuguese generally wants the article here:

• A chave está embaixo da mesa.

Without a, it sounds incomplete in most everyday situations. Portuguese uses articles more regularly than English.

Está is the third-person singular present tense form of estar.

It can mean:

• is
• are (in other forms)
• am (in other forms)

Here it matches a chave, which is singular:

• eu estou
• você/ele/ela está
• nós estamos
• eles/elas estão

So a chave está = the key is

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

• está = a form of the verb estar
• esta without the accent is usually a demonstrative, meaning this

So the accent matters because it helps distinguish different words.

Yes, but it is less common in everyday speech.

• sob a mesa also means under the table
• embaixo da mesa sounds more natural and conversational in Brazilian Portuguese

So both are correct, but embaixo da mesa is the more everyday choice.

In Brazilian Portuguese, chave is pronounced roughly like SHAH-vee.

A simple guide:

• ch sounds like sh
• a sounds like ah
• ve sounds like vee with a light v

And the whole sentence is roughly:

A chave está embaixo da mesa
≈ ah SHAH-vee es-TAH em-BAI-shoo dah ME-zah

This is only an approximation, but it can help at the beginning.