Este sofá é ainda mais confortável do que a cadeira.

Este means this. In traditional grammar, este refers to something near the speaker, while esse refers to something near the listener or something already mentioned.

So este sofá = this sofa.

In everyday Brazilian Portuguese, though, many people use esse where older grammar would prefer este. So in real life, este sofá and esse sofá can both be heard, but este is still perfectly correct and often appears in careful writing or teaching examples.

Here, é comes from ser, not estar.

Portuguese usually uses ser for a more general or inherent characteristic:

• O sofá é confortável. = The sofa is comfortable.

Use estar more for a temporary state or how something feels at the moment:

• O sofá está confortável hoje. = The sofa feels comfortable today.

In this sentence, the speaker is comparing the sofa and the chair as objects in general, so é is the natural choice.

Yes, ainda often means still, but here it has a different use.

In ainda mais, it means something like:

• even more
• still more
• even

So:

• mais confortável = more comfortable
• ainda mais confortável = even more comfortable

If you remove ainda, the sentence still works:

• Este sofá é mais confortável do que a cadeira.

That just sounds a little less emphatic.

In Portuguese, most adjectives form the comparative with mais + adjective, just like more + adjective in English.

So:

• confortável = comfortable
• mais confortável = more comfortable

Portuguese does not normally create forms like comfortabler. Instead, it uses mais.

This is true for most adjectives:

• mais bonito = more beautiful
• mais fácil = easier / more easy
• mais interessante = more interesting

Do que here means than in a comparison.

So:

• mais confortável do que a cadeira = more comfortable than the chair

After comparatives like mais and menos, Portuguese often uses:

• que
• or do que

Both are common in many cases.

Important: in this sentence, do que is a comparison phrase. You should not try to interpret do here as agreeing with sofá or with cadeira. It stays do que as part of the expression meaning than.

Yes. Both are used.

So these are both natural:

• Este sofá é mais confortável que a cadeira.
• Este sofá é mais confortável do que a cadeira.

Do que can sound a little clearer or a little more formal/careful, but que alone is very common in speech.

Portuguese often uses definite articles more than English does.

Here, a cadeira means the chair. It sounds natural if the speaker has a particular chair in mind, or if the chair is already understood from context.

Without the article, cadeira would sound less natural in this specific comparison.

So:

• o sofá = the sofa
• a cadeira = the chair

Also notice that cadeira is feminine, so the article is a.

Because confortável describes sofá, not cadeira.

The structure is:

• Este sofá = subject
• é = is
• ainda mais confortável = is even more comfortable
• do que a cadeira = than the chair

So the adjective belongs to sofá.

Also, confortável has the same form for masculine and feminine in the singular:

• o sofá confortável
• a cadeira confortável

It only clearly changes in the plural:

• sofás confortáveis
• cadeiras confortáveis

The accents are part of the correct spelling and pronunciation.

• é: this is the verb is. The accent shows that it is pronounced like eh and also distinguishes it from e, which means and.
• sofá: the accent shows the stress is on the last syllable: so-FÁ.
• confortável: the accent marks the stressed syllable: con-for-TÁ-vel.

Portuguese accents are very important because they often show where the stress goes and can also distinguish different words.

É ainda mais confortável is the most natural order here.

The sequence works like this:

• é = is
• ainda = even
• mais confortável = more comfortable

So the sentence builds naturally as:

• This sofa is even more comfortable than the chair.

You might hear other word orders in special contexts, but for a learner, é ainda mais confortável is the best and safest pattern to use.

The sentence as written is correct and natural. In everyday Brazilian Portuguese, a speaker might also say:

• Esse sofá é ainda mais confortável que a cadeira.
• Esse sofá é bem mais confortável que a cadeira.

A few notes:

• esse is very common in speech where textbooks may show este
• que alone is very common instead of do que
• bem mais means much more or a lot more

So the textbook sentence is good Portuguese, but spoken Brazilian Portuguese often uses slightly simpler forms.