O professor explicou que uma ultrapassagem mal feita pode ser mais perigosa do que parece.

In this sentence, ultrapassagem means an overtaking manoeuvre or the act of passing another vehicle on the road.

So uma ultrapassagem mal feita is:

• a badly done overtake
• an unsafe overtaking manoeuvre
• overtaking done badly

In Portugal, ultrapassagem is a very common word in driving and road-safety contexts.

Because here Portuguese uses a noun phrase, not a verb phrase.

• ultrapassar mal = to overtake badly
• uma ultrapassagem mal feita = a badly executed overtake / overtaking manoeuvre

The sentence is talking about the manoeuvre itself as a thing, so the noun ultrapassagem is used.

Because feita agrees with ultrapassagem, which is a feminine noun.

• ultrapassagem = feminine
• so the adjective-like form must also be feminine: feita
• masculine would be feito

Compare:

• um trabalho mal feito = a badly done job
• uma ultrapassagem mal feita = a badly done overtake

Yes, effectively it works like an adjective here.

Literally:

• feita is the past participle of fazer = done / made
• mal feita = badly done

So uma ultrapassagem mal feita literally means an overtaking manoeuvre badly done.

This structure is very common in Portuguese:

• um texto bem escrito = a well-written text
• uma decisão mal pensada = a badly thought-out decision
• uma ultrapassagem mal feita = a badly executed overtake

This is a very common learner question.

• mal = badly or poorly → adverb
• mau / má = bad → adjective

Here, mal modifies feita, meaning done badly, so the adverb is needed.

Compare:

• uma ultrapassagem mal feita = an overtake badly done
• uma má ultrapassagem = a bad overtake

Both are possible, but they are not identical in structure:

• má describes the noun directly
• mal describes how it was done

Because pode ser means can be or may be, which makes the statement less absolute.

• é mais perigosa = is more dangerous
• pode ser mais perigosa = can be more dangerous / may be more dangerous

So the sentence is saying that a badly done overtake has the potential to be more dangerous than it looks.

Because it agrees with ultrapassagem, which is feminine.

• uma ultrapassagem → feminine singular
• therefore perigosa must also be feminine singular

Compare:

• um carro perigoso = a dangerous car
• uma ultrapassagem perigosa = a dangerous overtake

Here do que means than.

It appears after a comparative structure:

• mais ... do que ... = more ... than ...
• menos ... do que ... = less ... than ...

So:

• mais perigosa do que parece = more dangerous than it seems

This is one of the standard ways Portuguese expresses comparison.

Portuguese often leaves out subject pronouns when they are understood from the context.

In English, you must say:

• than it seems

In Portuguese, the it is not said explicitly here:

• do que parece

The understood idea is:

• than it seems
• or more literally, than [it] appears

The subject is implicit.

Parece comes from parecer, which usually means:

• to seem
• to appear

Here:

• parece = it seems

So mais perigosa do que parece means:

• more dangerous than it seems
• more dangerous than it appears

Because the implied subject is singular.

In English, the hidden subject is it:

• than it seems

Portuguese does not need to say isso or ela here, but the verb still stays singular:

• parece = it seems

If the subject were plural, the verb would change:

• mais perigosas do que parecem = more dangerous than they seem

Explicou is the pretérito perfeito simples in Portuguese, which usually corresponds to the simple past in English.

• explicar = to explain
• explicou = explained

So:

• O professor explicou que... = The teacher explained that...

It refers to a completed action in the past.

Because que introduces a subordinate clause, just like that in English.

• explicou que... = explained that...

So the sentence structure is:

• O professor explicou = The teacher explained
• que uma ultrapassagem... = that an overtaking manoeuvre...

This use of que is extremely common in Portuguese.

In Portuguese, professor often means teacher in a broad sense, not only a university professor.

So O professor explicou... can naturally mean:

• The teacher explained...

Context determines whether it is specifically a schoolteacher, lecturer, or professor.

Because Portuguese uses definite articles very naturally before nouns when referring to a specific person or thing.

Here:

• O professor = the teacher

It refers to a particular teacher already known in the context.

Portuguese generally uses articles more often than English does, so this is very normal.

Because uma means a/an, and it presents the idea as one example of such a manoeuvre, not a specific previously identified one.

• uma ultrapassagem = an overtaking manoeuvre / a certain overtake
• a ultrapassagem = the overtaking manoeuvre

So the sentence is making a general point:

• a badly done overtake can be more dangerous than it seems

Yes, completely normal.

Portuguese usually places descriptive adjectives and participle-based modifiers after the noun:

• um carro novo = a new car
• um texto mal escrito = a badly written text
• uma ultrapassagem mal feita = a badly done overtake

So the word order is standard and natural.

Yes, many speakers also say mais perigosa que parece, and it is widely understood.

However, mais ... do que ... is very common and often taught as the standard comparative pattern:

• mais perigosa do que parece

So for a learner, do que is a very safe and natural choice.

A native speaker would probably process it like this:

• O professor explicou = The teacher explained
• que = that
• uma ultrapassagem mal feita = a badly executed overtake
• pode ser = can be
• mais perigosa = more dangerous
• do que parece = than it seems

So grammatically, it is a very normal sentence with:

1. a main clause: O professor explicou
2. a subordinate clause introduced by que
3. a comparative structure: mais ... do que ...

In European Portuguese, the pronunciation is approximately:

• ul-tra-pas-SA-zhem

A rough guide for an English speaker:

• ultra as in ool-tra
• passa with the stress on sa
• gem sounds more like zhẽj or zheng with a nasal quality, depending on accent

The g in -gem sounds like the s in measure.

You do not need to pronounce it exactly like the spelling suggests in English. The ending -agem in Portuguese often has that soft zh sound.