Agarra bien el volante cuando la carretera está mojada.

Agarra is the tú imperative of agarrar, so it means grab/hold as a command or instruction.

• agarras = you grab/you hold (present indicative)
• agarra = grab!/hold! (command to tú)

So the sentence is giving advice or an instruction, not just describing what someone does.

It is understood in Spain, but in many parts of Spain people would more often say coge bien el volante or sujeta bien el volante.

A few useful nuances:

• agarrar = to grab, seize, hold
• coger = very common in Spain for take/grab/hold
• sujetar = to hold firmly, secure

So your sentence is correct, but for specifically Spain Spanish, coge bien el volante cuando la carretera está mojada may sound a bit more idiomatic.

Here bien does not simply mean well in a general sense. In this sentence it means something more like:

• firmly
• properly
• securely

So agarra bien el volante means hold the steering wheel firmly/properly.

Spanish often uses the definite article (el, la, los, las) instead of a possessive like my/your when the owner is obvious from context.

Here, if someone is driving, it is clear which steering wheel is meant, so el volante sounds natural.

Compare:

• Agarra bien el volante = Hold the steering wheel firmly.
• Agarra bien tu volante = Hold your steering wheel firmly.

The second version is not wrong, but it sounds more marked, as if you are contrasting your steering wheel with someone else’s.

Because this sentence talks about a general situation or habitual condition: whenever the road is wet.

So Spanish uses the indicative:

• cuando la carretera está mojada = when/whenever the road is wet

If you were talking about a future situation that has not happened yet, Spanish often uses the subjunctive:

• Cuando la carretera esté mojada, conduce con más cuidado. = When the road is wet / when the road becomes wet, drive more carefully.

In your sentence, the speaker is giving general advice, so está is the normal choice.

Because mojada has to agree with la carretera, which is a feminine singular noun.

Agreement works like this:

• el camino mojado
• la carretera mojada
• los caminos mojados
• las carreteras mojadas

So mojada is feminine singular to match carretera.

They are not the same:

• carretera = road, highway, main road between places
• calle = street, usually in a town or city

So la carretera está mojada suggests a road you are driving on, not necessarily a city street.

Yes, both are possible, but the most natural version here is agarra bien el volante.

Why?

• agarra bien el volante puts bien close to the verb, so it sounds like hold it properly/firmly
• agarra el volante bien is possible, but can sound slightly more emphatic or a bit less neutral depending on context

In everyday advice, agarra bien el volante flows more naturally.

It is omitted because Spanish often leaves out subject pronouns when they are clear from the verb form or the context.

So:

• Agarra bien el volante = (Tú) agarra bien el volante

In commands, leaving out tú is especially normal. You only add tú for emphasis, contrast, or clarity:

• Tú agarra bien el volante; yo me encargo del resto.

By itself, agarra can also mean he/she grabs or you grab in some formal address systems, but in this sentence the context makes it clearly a command.

Sentence openings like this are very commonly interpreted as imperatives:

• Agarra bien el volante...
• Mira esto...
• Escucha...

So here a native speaker understands it as advice or instruction: Hold the steering wheel firmly.

Here volante means steering wheel.

Be aware that volante can have other meanings in other contexts, such as:

• flyer/leaflet
• midfield player in some sports contexts
• steering wheel in driving contexts

In this sentence, because of carretera and mojada, the driving meaning is obvious.

It can be either, depending on tone and context.

The imperative in Spanish can express:

• a direct order
• advice
• an instruction
• a safety recommendation

In this sentence, it most naturally sounds like advice/instruction, especially because it is about safe driving. It is similar to saying:

• Make sure you hold the steering wheel firmly when the road is wet.

No. A command in Spanish does not need exclamation marks.

So all of these are possible:

• Agarra bien el volante cuando la carretera está mojada.
• ¡Agarra bien el volante cuando la carretera está mojada!

Without exclamation marks, it sounds like normal advice or instruction. With them, it sounds more urgent or forceful.