É melhor sublinhar as frases mais importantes do que destacar tudo.

É melhor + infinitive is a very common Portuguese pattern meaning It is better to + verb.

So:

É melhor sublinhar... = It is better to underline...

This is an impersonal construction. Portuguese does not need a subject like it here. The é is just the 3rd-person singular of ser, used in a general statement.

You can use the same pattern in many other sentences:

É melhor esperar. = It is better to wait.
É melhor sair agora. = It is better to leave now.

The accent is important because é and e are different words:

• é = is
• e = and

So in this sentence, É melhor means It is better. Without the accent, E melhor would mean And better, which is not the intended meaning.

They are in the infinitive because after é melhor, Portuguese normally uses a verb in its basic form.

So:

• sublinhar = to underline
• destacar = to highlight / emphasize / make stand out

This works like English It is better to underline... than to highlight...

Portuguese often leaves the verbs in infinitive form in this structure instead of conjugating them.

Here do que means than in a comparison.

So the structure is:

melhor ... do que ... = better ... than ...

In this sentence:

É melhor sublinhar as frases mais importantes do que destacar tudo.
= It is better to underline the most important sentences than to highlight everything.

In everyday Brazilian Portuguese, you may also hear melhor ... que ..., with just que. Both are common, but do que is very standard and clear.

Yes, this is a very common point of confusion.

In Portuguese, frase often means a sentence or a complete statement, not just an English-style phrase.

So:

• Portuguese frase often = English sentence
• English phrase is not always the same as Portuguese frase

Depending on context, Portuguese frase can sometimes also mean a short expression, but in a study/text-marking context, as frases mais importantes is very naturally understood as the most important sentences.

This noun phrase breaks down like this:

• as = the
• frases = sentences
• mais importantes = most important

Portuguese adjectives usually come after the noun, so:

frases importantes = important sentences

When you add mais, you get:

frases mais importantes = more important / most important sentences

And with as, the whole phrase usually means:

as frases mais importantes = the most important sentences

By itself, mais importantes can mean either more important or most important, depending on context.

In this sentence, because we have as frases mais importantes, it is understood as the most important sentences.

Compare:

• frases mais importantes que estas = sentences more important than these
• as frases mais importantes do texto = the most important sentences in the text

So the article as helps push the meaning toward the most important.

They are related, but not identical.

• sublinhar literally means to underline
• destacar means to highlight, emphasize, or make stand out

In this sentence, the contrast is something like:

• be selective and underline the key sentences
• instead of highlighting everything

In Brazilian Portuguese, if you mean using a highlighter pen specifically, people also often say grifar.

So:

• sublinhar = underline
• grifar = highlight with a marker
• destacar = broader idea of making something stand out

Tudo means everything.

It is used here in a general way:

destacar tudo = to highlight everything

If you said todas, it would need to agree with a plural noun that is stated or clearly understood:

• todas as frases = all the sentences

So the sentence could also be rephrased as:

É melhor sublinhar as frases mais importantes do que destacar todas as frases.

But destacar tudo sounds more natural and more general, like marking every single thing.

The tricky part is lh, which is a very common Portuguese sound.

In Brazilian Portuguese, lh sounds somewhat like the lli in million for many English speakers, though not exactly.

So sublinhar is approximately:

soo-bleen-YAR

A rough breakdown:

• sub → sounds like soob or sub, depending on speaker
• linh → the lh is the special sound
• ar → ends like ar in many Portuguese infinitives, with stress on the last syllable here

The stress is on the last syllable:

subliNHAR

If the lh is difficult at first, that is completely normal. It also appears in words like:

• filho = son
• trabalho = work
• mulher does not have lh, so do not copy that sound there

Yes, in informal spoken Brazilian Portuguese, people often shorten this kind of sentence:

Melhor sublinhar as frases mais importantes do que destacar tudo.

That sounds natural in conversation and means basically the same thing.

Still, É melhor... is the fuller, more neutral form, and it is a very good pattern for learners to know and use.