Hay que fijarse en la fecha de caducidad antes de comprar yogur.

Hay que + infinitive is a very common Spanish structure meaning it is necessary to..., one must..., or you have to... in a general, impersonal way.

So:

• Hay que fijarse... = You have to pay attention... / One must look... / It’s necessary to check...

It does not refer to one specific person. It means this is something people should do in general.

Compare:

• Hay que fijarse en la fecha... = People in general should check the date.
• Tienes que fijarte en la fecha... = You have to check the date.

So hay que is used because the advice is general.

In the expression hay que, Spanish always uses hay, which comes from haber.

This is a fixed impersonal structure:

• hay que + infinitive

It does not change according to person:

• Hay que estudiar
• Hay que salir temprano
• Hay que fijarse en la fecha de caducidad

Even if in English we translate it as you have to or we have to, Spanish still keeps hay que for the general meaning.

Fijarse en means something like:

• to pay attention to
• to notice
• to look carefully at
• to check

In this sentence, it means you should pay attention to or check the expiry date.

Examples:

• Fíjate en el precio. = Pay attention to the price.
• No me fijé en su nombre. = I didn’t notice his name.
• Hay que fijarse en la fecha de caducidad. = You have to check the expiry date.

Because fijar and fijarse are different verbs in practice.

• fijar usually means to fix, to set, or to attach
• fijarse en means to notice or to pay attention to

Examples:

• Fijar una fecha = to set a date
• Fijarse en una fecha = to notice / pay attention to a date

So in your sentence, the reflexive form is essential.
You are not setting the expiry date; you are looking at it carefully.

Because the verb is normally fijarse en algo/alguien.

So the full pattern is:

• fijarse en + noun
• fijarse en + pronoun

Examples:

• Me fijo en los detalles. = I pay attention to the details.
• Fíjate en eso. = Look at that / Pay attention to that.
• Hay que fijarse en la fecha...

This en is simply part of how the verb works. It is not optional here.

La fecha de caducidad is the standard Spanish term for an expiry date or use-by date, especially on food that should not be consumed after that date.

Breakdown:

• fecha = date
• caducidad = expiry, expiration

So literally it is the date of expiry.

In Spain, this is the usual phrase on food packaging when the product should not be eaten after that date.

Not exactly in usage.

For Spain, fecha de caducidad is the normal expression.
In many parts of Latin America, fecha de vencimiento is also very common.

So:

• Spain: usually fecha de caducidad
• Many Latin American countries: often fecha de vencimiento

Both refer to the expiry date, but caducidad is the most natural choice for Spain, which matches your sentence.

This is a very useful real-world distinction in Spain.

• Fecha de caducidad = use-by date
  After this date, the food may no longer be safe to eat.
• Consumo preferente = best before date
  After this date, the food may lose quality, but it is not automatically unsafe.

So if the sentence says:

• Hay que fijarse en la fecha de caducidad...

it suggests checking the date for safety, not just quality.

Because after antes de, Spanish uses the infinitive when the subject is the same or when the subject is general/unspecified.

So:

• antes de comprar = before buying

This is the normal structure.

You use antes de que + subjunctive when there is a new, specific subject after it.

Compare:

• Hay que fijarse en la fecha antes de comprar yogur.
  = You should check the date before buying yogurt.
• Hay que fijarse en la fecha antes de que lo compren.
  = You should check the date before they buy it.

So in your sentence, antes de comprar is correct because no new subject is introduced.

It is in the infinitive because it comes after the preposition de in the expression antes de + infinitive.

This is very common in Spanish:

• antes de salir = before leaving
• después de comer = after eating
• sin mirar = without looking
• al llegar = upon arriving

So:

• antes de comprar yogur = before buying yogurt

Because comprar yogur here is talking about yogurt in a general sense, not a specific yogurt already known to the speaker and listener.

So:

• comprar yogur = to buy yogurt / to buy some yogurt
• comprar el yogur = to buy the yogurt, a specific one

In general statements, Spanish often leaves out the article with nouns like this.

Compare:

• Voy a comprar pan. = I’m going to buy bread.
• Hay que comparar precios antes de comprar coche. = You should compare prices before buying a car.
• ...antes de comprar yogur. = ...before buying yogurt.

If you said el yogur, it would sound more specific.

Yes. Both yogur and yogurt are accepted spellings.

In Spain, yogur is very common and is often preferred in standard written Spanish.
You may also see yogurt on packaging or in everyday use.

So in this sentence, yogur is completely natural.

Because Spanish often uses the definite article where English might or might not use one.

Here, la fecha de caducidad means the expiry date, referring to the date on the product package.

Spanish commonly says:

• Mira la etiqueta = Look at the label
• Revisa el precio = Check the price
• Fíjate en la fecha de caducidad = Check the expiry date

Using la sounds natural because it refers to the relevant date on the product.

Yes, but the meaning is slightly different.

• mirar = to look at
• fijarse en = to pay attention to / to notice / to check carefully

So:

• Hay que mirar la fecha de caducidad... = You have to look at the expiry date.
• Hay que fijarse en la fecha de caducidad... = You have to pay attention to / check the expiry date.

Fijarse en sounds a bit more careful and deliberate, which fits this context very well.

With an infinitive, the reflexive pronoun is normally attached to the end:

• fijarse

That is what you have here:

• Hay que fijarse en la fecha...

If the verb were conjugated, the pronoun would usually go before it:

• Me fijo en la fecha.
• Te fijas en la fecha.
• Se fija en la fecha.

So in your sentence, attaching -se to the infinitive is exactly right.

Yes, very much so.

Several parts sound especially natural for Spain:

• fecha de caducidad is the usual Spain Spanish term
• yogur is a standard Spain spelling and word choice
• hay que is a very common everyday way to give general advice

So the whole sentence sounds like natural, standard Spanish from Spain.