Hay que tener cuidado cuando el suelo está mojado.

Hay que + infinitive is a very common Spanish structure meaning one must, you have to, or it is necessary to do something.

So:

• Hay que tener cuidado = You have to be careful / One must be careful

It is an impersonal structure, so it does not name a specific person. It talks about people in general.

Compare:

• Hay que estudiar = You have to study / One must study
• Hay que salir temprano = You need to leave early

In your sentence, it gives general advice rather than telling one particular person what to do.

Both can express obligation, but they are used differently.

• Hay que + infinitive = general obligation, for people in general
• Tener que + infinitive = obligation for a specific person

Examples:

• Hay que tener cuidado = You have to be careful / People have to be careful
• Tienes que tener cuidado = You have to be careful specifically addressing you
• Tenemos que tener cuidado = We have to be careful

So the sentence uses hay que because it sounds like a general warning, not a personal command.

In Spanish, tener cuidado is the normal expression for to be careful or to take care.

So:

• tener cuidado = to be careful

Spanish often uses expressions with tener where English uses to be.

For example:

• tener hambre = to be hungry
• tener miedo = to be afraid
• tener razón = to be right
• tener cuidado = to be careful

A single word ¡Cuidado! also exists, but that is usually an exclamation meaning Careful! or Watch out!

So:

• ¡Cuidado! = Watch out!
• Hay que tener cuidado = You have to be careful

Here suelo can mean either floor or ground, depending on context.

• indoors: floor
• outdoors: ground

So the sentence could mean:

• You have to be careful when the floor is wet
• You have to be careful when the ground is wet

In Spain, suelo is a very normal word for the surface you walk on. It is often preferred over piso when talking about a physical floor, because in Spain piso very often means flat/apartment.

Spanish uses estar for states or conditions, especially temporary ones. A wet floor is normally a temporary condition, so Spanish says:

• el suelo está mojado = the floor is wet

Using ser here would sound wrong in normal Spanish because being wet is not seen as an essential characteristic of the floor.

A useful rule:

• ser = what something is
• estar = how something is, or what state it is in

More examples:

• La puerta está abierta = The door is open
• La comida está fría = The food is cold
• La calle está mojada = The street is wet

Mojado is an adjective, and in Spanish adjectives usually agree with the noun they describe.

Here the noun is:

• el suelo — masculine singular

So the adjective must also be masculine singular:

• mojado

Compare:

• el suelo mojado = masculine singular
• la calle mojada = feminine singular
• los suelos mojados = masculine plural
• las calles mojadas = feminine plural

That is why the sentence says el suelo está mojado.

This is a very common question.

Spanish uses:

• indicative after cuando when referring to something real, habitual, or already known
• subjunctive after cuando when referring to a future or not-yet-real situation

In your sentence, the meaning is general and habitual:

• Hay que tener cuidado cuando el suelo está mojado.
• You have to be careful when the floor is wet.

This means whenever the floor is wet, as a general fact.

But if you were talking about a future situation, you would usually use the subjunctive:

• Hay que tener cuidado cuando el suelo esté mojado.

That would sound more like You have to be careful when the floor is wet / when it becomes wet in a future or possible situation.

So both can exist, but the meaning changes slightly:

• cuando está mojado = whenever it is wet, as a general rule
• cuando esté mojado = when it is wet in the future / when that situation occurs

Not normally, unless the context already makes it completely clear what it is.

In Spanish, subject pronouns are often omitted, but nouns are not omitted in the same way if the reference is unclear. In this sentence, está mojado needs a clear subject:

• el suelo está mojado

If you are already talking about the floor, then you might hear:

• Hay que tener cuidado cuando está mojado

But on its own, this sounds incomplete or ambiguous, because the listener may wonder: what is wet?

So in most cases, keeping el suelo is the natural choice.

Grammatically it is when, but in this sentence the idea is often closer to whenever.

• Hay que tener cuidado cuando el suelo está mojado.

This does not usually refer to just one single moment. It gives a general rule:

• You have to be careful whenever the floor is wet

Spanish often uses cuando in this broader habitual sense without needing a separate word for whenever.

It depends on the variety of Spanish.

In much of Latin America, piso can mean floor, so:

• cuando el piso está mojado is very natural there

In Spain, piso very often means flat/apartment, so suelo is usually clearer for floor as a surface.

Since you are learning Spanish from Spain, suelo is the better choice here.

It works mainly as a general warning or general piece of advice.

Because it uses hay que, it sounds impersonal and broad:

• People should be careful when the floor is wet

It is not as direct as:

• Ten cuidado cuando el suelo está mojado = Be careful when the floor is wet

And it is less formal and less forceful than something like:

• Se debe tener cuidado cuando el suelo está mojado = Care must be taken when the floor is wet

So the original sentence is natural, everyday Spanish for giving a general caution.