É perigoso atravessar a ponte quando o chão está molhado.

This is an impersonal structure, very common in Portuguese.

• É perigoso... = It is dangerous...
• The it in English is only a dummy subject. Portuguese often does not use a dummy subject here.

So:

• É perigoso atravessar a ponte.
• Literally: Is dangerous to cross the bridge
• Natural English: It is dangerous to cross the bridge

There is no separate word for it in this sentence.

Because after expressions like É perigoso, Portuguese often uses the infinitive to talk about an action in a general way.

So:

• É perigoso atravessar a ponte.
• It is dangerous to cross the bridge.

Here, atravessar means to cross.

This is the same idea as in:

• É fácil aprender. = It is easy to learn.
• É importante descansar. = It is important to rest.

Because perigoso is not describing a ponte directly. It is describing the action/situation of crossing the bridge.

So the idea is:

• Crossing the bridge is dangerous.

In this kind of impersonal sentence, Portuguese normally uses the adjective in the masculine singular form:

• É perigoso
• É difícil
• É importante

If you were describing the bridge itself, then it would change:

• A ponte é perigosa. = The bridge is dangerous.

That is a different sentence.

Portuguese uses definite articles more often than English.

So where English may say:

• cross the bridge
• when the ground is wet

Portuguese naturally says:

• atravessar a ponte
• quando o chão está molhado

This is completely normal. In Portuguese, nouns very often appear with an article, especially when talking about a specific thing or a general known thing in the situation.

Chão usually means ground or floor.

In this sentence, it refers to the surface you are walking on. So depending on context, English might think of it as:

• the ground
• the floor
• the surface of the bridge

That is why the sentence makes sense: if the walking surface is wet, crossing the bridge is dangerous.

In some contexts, a Portuguese speaker might also say:

• o piso = the floor/surface
• a ponte está molhada = the bridge is wet

But o chão is natural and understandable.

Because Portuguese uses estar for a temporary state or condition, and ser for something more permanent or defining.

Here, the ground is wet at that moment / in that situation, so:

• o chão está molhado = the ground is wet

Compare:

• está molhado = it is wet now
• é molhado = it is wet by nature / it is a wet kind of thing

So está is the correct choice here.

Because it agrees with o chão, which is a masculine singular noun.

• o chão → masculine singular
• molhado → masculine singular

If the noun were feminine, the adjective would change:

• a ponte está molhada = the bridge is wet

Agreement is very important in Portuguese.

Quando means when, while se means if.

In this sentence, quando o chão está molhado suggests a situation that happens whenever that condition is true:

• when the ground is wet

It sounds like a general statement:

• whenever the ground is wet, crossing the bridge is dangerous.

If you said:

• É perigoso atravessar a ponte se o chão estiver molhado.

that would mean:

• It is dangerous to cross the bridge if the ground is wet.

That is also possible, but the nuance is slightly different.
Quando sounds more like a general repeated situation; se sounds more like a condition.

Yes. Portuguese is flexible here.

You can say:

• É perigoso atravessar a ponte quando o chão está molhado.

or:

• Quando o chão está molhado, é perigoso atravessar a ponte.

Both are correct. The second version puts more emphasis on the condition first:

• When the ground is wet, it is dangerous to cross the bridge.

Yes, absolutely.

These two are both correct:

• É perigoso atravessar a ponte.
• Atravessar a ponte é perigoso.

They mean the same thing.

The first version is often a little more natural in everyday Portuguese, because impersonal É + adjective + infinitive is very common.

Sometimes yes, but atravessar is the most natural verb here.

• atravessar a ponte = to cross the bridge
• cruzar can also mean to cross, but it is often used in slightly different contexts too

For example:

• atravessar a rua = cross the street
• cruzar os braços = cross your arms
• cruzar a fronteira = cross the border

With a bridge, atravessar a ponte sounds very natural and standard.

Chão is one of the trickier words for English speakers.

A rough guide:

• ch sounds like sh
• ão is nasal, and English does not have an exact equivalent

So chão sounds roughly like shão, with a nasal ending.

Important points:

• it is not like English ch in chair
• the ão should be nasal, not a plain ow

Also, in European Portuguese, nasal sounds can be more closed and less exaggerated than learners expect.

The lh sound is special in Portuguese.

In molhado, it sounds roughly like the lli in some pronunciations of million, but that is only an approximation.

So:

• mo-lha-do

The middle sound is not a normal English l.

If you say a plain l, people may still understand you, but the real Portuguese sound is softer and more palatal.

Usually it sounds like a specific bridge or a bridge already known from the context, because of a ponte:

• the bridge

If you wanted a more general statement, you might say something like:

• É perigoso atravessar pontes quando o chão está molhado.
• It is dangerous to cross bridges when the surface is wet.

But in normal conversation, a ponte can still be used when the bridge is obvious from the situation.