Segura o volante com firmeza ao atravessar a ponte.

Here it is most naturally understood as a command: Segura o volante... = Hold the steering wheel...

Why?

• In European Portuguese, segura is the affirmative imperative form for tu of segurar.
• The surrounding context sounds like an instruction, so a command is the normal reading.

It is true that segura can also be the present indicative form meaning he/she holds or you hold in some contexts, but in this sentence the instruction-style wording makes the imperative interpretation the obvious one.

A more formal version would be:

• Segure o volante com firmeza... = command to você / o senhor / a senhora

It is understood, not stated.

Portuguese often leaves out subject pronouns when the verb form already shows who the subject is. In this sentence, segura tells us the implied subject is tu:

• (Tu) segura o volante com firmeza...

This is very normal in Portuguese. English usually needs you, but Portuguese often does not.

Portuguese uses definite articles much more often than English.

So o volante literally means the steering wheel, but in English we might simply say Hold the steering wheel or sometimes even just Hold the wheel.

In Portuguese, using the article here sounds natural because:

• the object is specific
• in the situation, everyone knows which steering wheel is meant

So o volante is exactly what you would expect.

Com firmeza means something like:

• firmly
• steadily
• with a firm grip
• with control

It does not necessarily mean with great force. It suggests stability and control rather than brute strength.

So:

• Segura o volante com firmeza = hold the steering wheel firmly / keep a steady grip on the steering wheel

This expression is very natural in Portuguese.

Ao atravessar is a very common Portuguese structure meaning:

• when crossing
• while crossing
• as you cross

So:

• ao atravessar a ponte = when/while crossing the bridge

For a learner, the easiest way to remember it is:

• ao + infinitive often expresses when doing or while doing something

Examples:

• Ao entrar, fecha a porta. = When entering, close the door.
• Ao conduzir, mantém a atenção. = When driving, keep your attention focused.

Both are possible, but they feel a little different.

Ao atravessar a ponte:

• is more compact
• sounds more like a general instruction
• is very common in advice, manuals, and safety language

Quando atravessas a ponte:

• is more like ordinary speech
• sounds more personal or conversational

So in an instructional sentence, ao + infinitive is especially natural.

You could also hear:

• Quando atravessares a ponte...

That uses the future subjunctive, which is also common in Portuguese for future situations after quando.

Because atravessar normally takes a direct object.

So:

• atravessar a ponte = to cross the bridge

No extra preposition is needed.

Compare:

• atravessar a ponte = cross the bridge
• passar pela ponte = go by / through / via the bridge

The verb atravessar directly treats a ponte as the thing being crossed.

The sentence breaks down like this:

• Segura = command verb, Hold
• o volante = direct object, the steering wheel
• com firmeza = manner, firmly / with a firm grip
• ao atravessar a ponte = time/circumstance, when/while crossing the bridge

So the structure is:

• Verb + object + manner + circumstance

This word order is very natural in Portuguese.

You could also say:

• Ao atravessar a ponte, segura o volante com firmeza.

That is also correct. It just puts the when crossing the bridge idea first for emphasis.

Yes, especially in an informal singular context.

In Portugal:

• Segura = informal command to tu
• Segure = more formal / polite singular
• Segurem = plural command

So the choice depends on who is being addressed.

For example:

• Segura o volante... = talking informally to one person
• Segure o volante... = more formal or polite
• Segurem o volante... = talking to more than one person

In instructions, signs, or manuals, Portuguese may also use other styles, but Segura is perfectly natural as an informal instruction.

The sentence itself is natural Portuguese in general, but one point that fits Portugal well is the use of the tu imperative:

• Segura

That is very normal in European Portuguese in informal speech.

A learner of Portuguese from Portugal should also notice that European Portuguese often:

• drops the subject pronoun
• uses articles very regularly
• likes compact structures such as ao + infinitive

So the sentence is a good example of several everyday European Portuguese patterns at once.