A chave está em cima da mesa.

A is the definite article for a feminine singular noun, so it means the.

• a chave = the key
• o livro = the book

In Portuguese, articles are used very often, sometimes more often than in English.

In Portuguese, every noun has a grammatical gender. Chave is a feminine noun, so it takes feminine words with it:

• a chave
• uma chave
• a chave pequena

This gender is grammatical, not biological. A key is not “female”; the noun simply belongs to the feminine class.

Portuguese has two main verbs for to be: ser and estar.

Here we use estar because the sentence gives the location of something:

• A chave está em cima da mesa. = location
• A mesa é grande. = permanent characteristic

A useful rule:

• ser = identity, definition, lasting characteristics
• estar = location, condition, temporary state

So for where something is, estar is the normal choice.

Está is the 3rd person singular of estar in the present tense.

The subject is a chave, which is singular, so the verb must also be singular:

• eu estou
• tu estás
• ele/ela está

Even though English uses is, Portuguese learners often need to remember that this is comes from estar, not ser, in this sentence.

Em cima de is a very common expression meaning on top of or simply on, depending on context.

So:

• em cima da mesa = on the table / on top of the table

It is a fixed expression:

• em cima de + noun

Examples:

• em cima da cama
• em cima do sofá
• em cima do carro

Because de + a contracts to da in Portuguese.

So:

• de + a mesa → da mesa

This kind of contraction is very common:

• de + o = do
• de + a = da
• em + a = na
• em + o = no

So em cima da mesa is the correct natural form, not em cima de a mesa.

Normally, em cima da mesa suggests the key is on the surface of the table.

If you want to say something is simply above the table, without necessarily touching it, Portuguese would more naturally use something like:

• por cima da mesa
• acima da mesa

So in this sentence, the usual interpretation is that the key is resting on the table.

Yes, sobre a mesa is possible and grammatically correct.

But the nuance can differ slightly:

• em cima da mesa = very common, concrete, physical location
• sobre a mesa = can also mean on the table, but depending on context it may sound a bit more formal or abstract

In everyday speech, em cima da mesa is extremely natural for physical placement.

The normal word order is:

subject + verb + place

So:

• A chave = subject
• está = verb
• em cima da mesa = location

This is the most neutral and natural order.

You can change it for emphasis, for example:

• Em cima da mesa está a chave.

But that sounds more marked or literary. For everyday Portuguese, the original order is the safest choice.

In European Portuguese, ch is pronounced like sh in shoe.

So chave begins with a sh sound.

A rough guide:

• chave ≈ SHAH-vuh (very approximate)

A few useful pronunciation notes:

• ch = sh
• the final -e in European Portuguese is often a reduced sound, not a strong ee
• v is a normal v sound

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

• está = he/she/it is
• esta = this (feminine)

So the accent matters because it distinguishes different words.

Not in normal Portuguese.

You normally need the article here:

• A chave está em cima da mesa.

Without a, it sounds ungrammatical in standard usage. Portuguese generally requires a determiner in this kind of sentence unless there is some special structure or context.