Este é o único problema.

Este is a demonstrative, meaning this.

A quick comparison:

• este = this
• esse = that
• isto = this/that without a noun after it

In Este é o único problema, este stands alone like this one or this in English.

A few useful notes for Brazilian Portuguese:

• In formal grammar, este is used for something closer to the speaker, and esse for something closer to the listener or something already mentioned.
• In everyday Brazilian speech, many people use esse where a textbook might prefer este.

So Este é o único problema is perfectly correct, but in casual speech you may also hear Esse é o único problema.

Because Portuguese usually uses ser for identity, definition, and classification, and estar for temporary states or conditions.

Here, the sentence is identifying something:

• Este é o único problema = it is being identified as the only problem

So é (from ser) is the natural choice.

Compare:

• Ele é médico. = He is a doctor.
• Ele está cansado. = He is tired.

In your sentence, the speaker is not describing a temporary condition. They are saying what this is.

Because o único problema means the only problem.

The article o is the masculine singular definite article, equivalent to the.

Breakdown:

• o = the
• único = only / single
• problema = problem

In Portuguese, you usually keep the definite article in expressions like the only...:

• o único problema = the only problem
• a única solução = the only solution

Without the article, the phrase would usually sound incomplete or unnatural in this context.

Because único must agree with problema, and problema is a masculine noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

• o único problema = the only problem
• a única solução = the only solution
• os únicos problemas = the only problems
• as únicas soluções = the only solutions

So the masculine noun problema requires único.

This is a very common learner question.

Although many Portuguese nouns ending in -a are feminine, some are masculine, especially words of Greek origin such as:

• o problema
• o sistema
• o tema
• o programa
• o idioma

So you have to learn problema as:

• o problema
• not a problema

That is why the sentence says o único problema.

They help with both pronunciation and, in one case, distinguishing words.

é

The accent in é shows that it is the verb is from ser.

• é = is
• e = and

So the accent is very important here because it changes the word completely.

único

The accent shows the stressed syllable:

• Ú-ni-co

Without the accent, the stress rules of Portuguese would suggest a different pronunciation.

So the accents are not optional; they are part of correct spelling.

In Portuguese, adjectives can come before or after the noun, depending on the adjective and the meaning.

With único, position matters:

• o único problema = the only problem
• um problema único = a unique problem

So in your sentence, único comes before the noun because it means only, not unique.

This is an important difference:

• o único amigo = the only friend
• um amigo único = a unique/special friend

Yes. In Brazilian Portuguese, Esse é o único problema is very common in natural speech.

The textbook distinction is often:

• este = this
• esse = that

But in modern spoken Brazilian Portuguese, esse often overlaps with este, especially in casual conversation.

So:

• Este é o único problema = correct, a bit more formal or careful
• Esse é o único problema = also very common and natural

A simple approximate pronunciation for Brazilian Portuguese is:

ES-cheh eh oo OO-nee-koo pro-BLEH-ma

A few details:

• Este in Brazil is often pronounced roughly like ES-chee or ES-tee, depending on accent
• é sounds like eh
• o before a consonant often sounds like oo
• único has stress on the first syllable: OO-ni-co
• problema has stress on ble: pro-BLE-ma

If you want a more syllable-by-syllable version:

• Es-te
• é
• o
• ú-ni-co
• pro-ble-ma

Yes, that is possible if the context already makes the subject clear.

• Este é o único problema. = This is the only problem.
• É o único problema. = It is the only problem. / That’s the only problem.

Portuguese often allows the subject to be omitted when it is obvious from context. But este is useful when you want to point something out clearly: this one.

So both can work, but este is more explicit.

The plural would be:

Estes são os únicos problemas.

Here is what changes:

• este → estes
• é → são
• o → os
• único → únicos
• problema → problemas

Everything that can agree in number changes to plural. This is very typical in Portuguese.

In this sentence, only is the best match.

• o único problema = the only problem

While único can sometimes be translated as single, in this structure it usually means that there is just one.

Also remember the position difference:

• o único problema = the only problem
• um problema único = a unique problem

So in your sentence, only is the key idea.