É perigoso circular na estrada sem capacete.

Portuguese uses ser (é) for general, permanent truths and estar (está) for temporary states.

• É perigoso circular… = It is inherently/always dangerous (a general rule).
• Hoje está perigoso circular… = Today it’s dangerous (because of temporary conditions like fog, ice, etc.).

This is the common impersonal pattern É + adjective + infinitive to make general statements:

• É fácil entender.
• É bom estudar.
  Here, É perigoso circular… means “It’s dangerous to …” in a general sense.
  If you want to specify who the subject is, European Portuguese allows the personal infinitive:
• Speaking to one person (tu): É perigoso circulares sem capacete.
• About us (nós): É perigoso circularmos sem capacete.
• About them/you-plural: É perigoso circularem sem capacete.

In European Portuguese, circular often means “to travel/move/drive/ride on roads,” especially in official or traffic contexts:

• Os carros não podem circular nesta rua. = Cars may not drive on this street.
  It’s not about spreading news or moving in circles; it’s the legal/traffic sense of being on the road network.

Yes, but with nuances:

• andar is everyday/neutral for “ride/go,” especially with a vehicle mentioned: andar de mota/de bicicleta.
  
  • Example: É perigoso andar de mota na estrada sem capacete.
• conduzir is “to drive/operate” a vehicle (car, motorbike, etc.).
  
  • Example: É perigoso conduzir uma mota sem capacete.
• circular sounds more formal/official (laws, road reports). Your sentence is perfectly idiomatic in Portugal, just a bit formal.

na = em + a (in/on + the, feminine singular).

• na estrada = “on the road.”
  Other forms: no (em + o), nas (em + as), nos (em + os).

Portuguese often uses the definite article for generic categories. na estrada here means “on roads” in general, not a specific road. This is normal:

• É perigoso nadar no mar. = It’s dangerous to swim in the sea (in general).

Yes:

• estrada = road (non-urban or interurban).
• autoestrada = motorway/freeway.
• rua = street (urban).
  The preposition works the same: na estrada / na autoestrada / na rua.

After sem, Portuguese typically drops the article when speaking generally: sem capacete = “without a helmet.”

• sem o capacete refers to a specific, known helmet (e.g., “without his helmet”).
• sem um capacete emphasizes “without a single/any helmet” (used when counting/insisting on at least one).

In the impersonal pattern É + adjective + infinitive, the adjective is normally masculine singular by default: É perigoso [infinitive].
If you turn the infinitive clause into a noun phrase, agreement changes:

• A condução sem capacete é perigosa. (feminine, because condução is feminine)

Yes. Both orders are fine:

• É perigoso circular na estrada sem capacete.
• Circular na estrada sem capacete é perigoso.
  The meaning and tone remain the same.

Your sentence is understood in Brazil, but everyday choices often differ:

• É perigoso andar de moto/dirigir moto na rodovia sem capacete.
  Brazil favors moto (Portugal: mota) and often uses dirigir or andar de moto. rodovia is common for highway (Portugal: autoestrada). circular exists but sounds more formal or bureaucratic.

No. circular is intransitive in this sense. You use a preposition:

• circular na/pela estrada, circular na cidade, circular a 120 km/h.

Commonly:

• É perigoso andar de mota na estrada sem capacete.
  Your original with circular is perfect, just a touch more formal/official.

Approximate guide (EP):

• É: “eh” (short, open)
• perigoso: “p’r-ree-GO-zu” (light r; g is a soft ‘g’ sound)
• circular: “seer-koo-LAR” (ci = ‘see’/‘sir’ sound)
• na estrada: roughly “nah sh-TRAH-dah” (initial s before t sounds like ‘sh’)
• sem: nasal “sẽ” (like “seng” without fully pronouncing the g)
• capacete: “kah-pah-SEH-te”
  Put together: “Eh p’r-ree-GO-zu seer-koo-LAR nah sh-TRAH-dah sɛ̃ kah-pah-SEH-te.”