Hay que usar el cuchillo con cuidado para no cortarse la mano.

Hay que + infinitive is an impersonal structure. It means something like it is necessary to..., you have to..., or one must... in a general sense.

So:

• Hay que usar el cuchillo con cuidado = You have to use the knife carefully / One must use the knife carefully

It does not refer to one specific person. It is more like a general rule or piece of advice.

By contrast:

• Tienes que usar el cuchillo con cuidado = You have to use the knife carefully

That speaks directly to one person.

So hay que is used because the sentence is stating a general caution, not giving an instruction to a specific individual.

Because after hay que, Spanish uses the infinitive.

Pattern:

• hay que + infinitive

Examples:

• Hay que estudiar. = You have to study.
• Hay que salir temprano. = You have to leave early.
• Hay que usar el cuchillo con cuidado. = You have to use the knife carefully.

So usar is in the infinitive because the grammar of hay que requires it.

In Spanish, articles like el, la, los, and las are used more often than in English.

So Spanish normally says:

• usar el cuchillo
• literally: to use the knife

Even when English might simply say use a knife or use the knife, Spanish often includes the article naturally.

Here, el cuchillo refers to the knife / a knife in that context. It sounds complete and normal in Spanish.

Con cuidado literally means with care.

In natural English, that usually becomes:

• carefully
• with caution
• being careful

So:

• usar el cuchillo con cuidado = to use the knife carefully

Spanish often uses a phrase with a noun where English might prefer an adverb.

Compare:

• con cuidado = carefully
• con calma = calmly
• con paciencia = patiently / with patience

Because cortarse means to cut oneself, while cortar just means to cut.

Compare:

• cortar la mano = to cut the hand
• cortarse la mano = to cut one’s hand / to cut oneself in the hand

In this sentence, the idea is that the person using the knife should avoid accidentally injuring themselves. That is why Spanish uses the reflexive form cortarse.

So:

• para no cortarse la mano = so as not to cut your hand / so you don’t cut your hand

Because when a reflexive pronoun goes with an infinitive, it is often attached to the end of the verb.

So:

• cortar + se → cortarse

This is completely normal with infinitives.

Examples:

• lavarse = to wash oneself
• ponerse = to put on
• hacerse daño = to hurt oneself

You could also sometimes see pronouns placed before a conjugated verb, but here the verb is an infinitive, so attaching se is the standard form.

Spanish often uses the definite article with body parts instead of possessive adjectives like mi, tu, or su, especially when it is already clear whose body part is meant.

So Spanish prefers:

• cortarse la mano
• literally: to cut oneself the hand

But the real meaning is:

• to cut one’s hand
• to cut your hand

Because se already shows that the action happens to the same person, Spanish does not usually need su here.

This is very common with body parts:

• Me duele la cabeza. = My head hurts.
• Se lavó las manos. = He/She washed his/her hands.
• No te toques la cara. = Don’t touch your face.

Because mano is an exception: it is a feminine noun.

So it is:

• la mano
• una mano
• las manos

Not:

• el mano

This is one of the most famous exceptions to the usual pattern that nouns ending in -o are masculine.

Para often means in order to or so that. When it is followed by no + infinitive, it means in order not to... or so as not to...

So:

• para no cortarse la mano = in order not to cut one’s hand

This part explains the purpose of being careful.

Structure:

• para + infinitive = in order to...
• para no + infinitive = in order not to...

Examples:

• Estudia para aprender. = Study in order to learn.
• Habla bajo para no despertar al bebé. = Speak quietly so as not to wake the baby.

Not necessarily. In this sentence, it usually means to cut one’s hand accidentally, not to cut the hand off.

Spanish cortar can mean to cut, and the exact meaning depends on context.

Here, because the sentence is about using a knife carefully, the natural meaning is:

• to cut your hand
• to injure your hand with the knife

If someone wanted to be very explicit about cutting off a hand, they would usually make that clearer in some other way.

Yes. That is exactly one of the effects of hay que.

The sentence can sound like:

• a general safety rule
• common advice
• an impersonal warning

So it is less direct than:

• Tienes que usar el cuchillo con cuidado...

and more like:

• You have to be careful when using a knife
• One must use the knife carefully

That impersonal tone is very common in instructions, notices, and general advice in Spanish.