En el sobre hay que escribir el nombre de la destinataria y también el del remitente.

Hay que + infinitive is an impersonal obligation structure. It means something like:

• one must
• you have to
• it is necessary to

So hay que escribir means it is necessary to write or you have to write.

It does not name a specific person who must do it. It is general, like giving instructions.

En el sobre means on the envelope or in/on the envelope, depending on context, but here it refers to what must be written on the outside of the envelope.

Spanish uses en in places where English may prefer on. So even though English often says on the envelope, Spanish commonly says en el sobre.

Also, en + el contracts to en el.

Because Spanish nouns have their own grammatical gender, which often does not match English.

Sobre is a masculine noun in Spanish, so it takes el:

• el sobre

That is why the sentence has En el sobre.

Yes, sobre has several meanings in Spanish. Here it is a noun, meaning envelope.

Compare:

• el sobre = the envelope
• sobre as a preposition = about, on, over, depending on context

In this sentence, the article el shows that sobre is being used as a noun.

Spanish often expresses possession with de rather than using a possessive adjective.

So:

• el nombre de la destinataria = the recipient’s name
• literally: the name of the female recipient

This is completely normal Spanish style, especially in formal or instructional language.

Because these words work differently.

• destinatario / destinataria changes form for masculine and feminine
• remitente is usually the same form for both genders; the article shows the gender

So:

• la destinataria = the female recipient
• el remitente = the male sender
• la remitente = the female sender

In your sentence, the recipient is specifically female, and the sender is presented as masculine.

El del remitente means the one belonging to the sender, and here el stands in for el nombre.

So the full idea is:

• el nombre de la destinataria y también el nombre del remitente

But Spanish often avoids repetition, so it shortens this to:

• el nombre de la destinataria y también el del remitente

This is very natural. In English, it is similar to saying:

• the recipient’s name and also the sender’s

where sender’s stands for sender’s name.

Yes. That would be completely correct.

Compare:

• ...y también el del remitente = more concise, avoids repetition
• ...y también el nombre del remitente = fuller, more explicit

Both are natural. The version in your sentence is just a bit more elegant and economical.

Because del remitente by itself would only mean of the sender. It would be missing the thing being referred to.

The el is necessary because it replaces el nombre:

• el del remitente = the one of the sender = the sender’s one/name

Without el, the structure feels incomplete here.

También means also or too.

Here it adds the second item that must be written:

• el nombre de la destinataria
• y también el del remitente

So the meaning is: write the recipient’s name, and also the sender’s.

Its placement here is very natural. Spanish can sometimes move también around, but in this sentence this position sounds clear and standard.

Yes, it sounds fairly neutral to formal, especially because of:

• the impersonal instruction hay que
• vocabulary like destinataria and remitente

This is the kind of sentence you might see in instructions, forms, school materials, or official explanations.

In casual speech, people might say something simpler, but this sentence is perfectly natural Spanish.