A opinião da professora é importante.

Because da professora means of the teacher or, more naturally in English, the teacher’s.

In Portuguese, possession is often expressed with de:

• a opinião da professora = the opinion of the teacher = the teacher’s opinion

If you said only A opinião professora, that would be incorrect.

Da is a contraction of de + a.

• de = of / from
• a = the (feminine singular)

So:

• de + a = da

Because professora is feminine singular, Portuguese uses da professora.

Compare:

• do professor = of the male teacher
• da professora = of the female teacher

Because it is a feminine noun in Portuguese, so it takes the feminine article a:

• a opinião

Grammatical gender in Portuguese does not always match any logical or natural gender. You simply learn each noun with its gender.

A good habit is to memorize nouns together with their article:

• a opinião
• a professora
• o professor

Portuguese often uses the definite article where English might omit it less often.

In this sentence, A opinião da professora means the teacher’s opinion as a complete noun phrase, so a is natural and expected.

Without the article, Opinião da professora é importante, it would sound unnatural in standard Portuguese.

Portuguese uses ser and estar differently.

Here, é importante uses ser because it describes a general or essential quality:

• A opinião da professora é importante. = The teacher’s opinion is important.

Estar is more often used for temporary states or conditions. So está importante would not sound right here.

Because importante is an adjective with the same form for masculine and feminine singular.

So you get:

• o livro importante
• a opinião importante

It does change for plural:

• o livro importante
• os livros importantes
• a opinião importante
• as opiniões importantes

So it agrees in number, but not in gender.

Yes. That is the most natural English translation.

Portuguese often uses de constructions where English uses apostrophe ’s:

• a opinião da professora = the teacher’s opinion
• o carro do professor = the teacher’s car

So even though the Portuguese structure is literally closer to the opinion of the teacher, the normal English translation is usually the teacher’s opinion.

The accents help show pronunciation and, sometimes, distinguish words.

• opinião has ão, a very common nasal ending in Portuguese.
• é is the verb is and carries an accent.

In opinião, the stress falls on the last syllable:

• o-pi-ni-ÃO

In é, the accent shows the stressed vowel sound:

• é = is

These accents are part of the correct spelling and should not be omitted in standard writing.

A rough pronunciation guide is:

ah oh-pee-nee-OWNG dah proh-feh-SO-rah eh eem-por-TAHN-chee

A few important points:

• opinião ends in a nasal ão sound
• professora is stressed on so
• importante is stressed on tan
• é sounds like eh

Pronunciation varies by region in Brazil, but this is a useful general approximation.

Yes, but the original order is the most neutral and natural:

• A opinião da professora é importante.

You may also see:

• É importante a opinião da professora.

This version is grammatical, but it sounds more emphatic or stylistically marked, almost like The teacher’s opinion is important with special emphasis on important.

For most learners, the original sentence is the best default pattern.

Here, professora refers to a specific female teacher, not the profession in general.

Because of da professora, the phrase means of the teacher or the teacher’s.

If you were talking about teachers in general, the structure would be different, for example:

• A opinião de uma professora... = A teacher’s opinion...
• A opinião das professoras... = The teachers’ opinion / the opinions of the teachers

So in this sentence, it is most naturally understood as a specific teacher.

Yes. That is the masculine version:

• A opinião da professora é importante. = The female teacher’s opinion is important.
• A opinião do professor é importante. = The male teacher’s opinion is important.

Here:

• do = de + o
• da = de + a

So the contraction changes to match the gender of the noun that follows.