En la segunda columna del formulario faltaba el apellido de la destinataria, así que tuve que volver a escribir la página entera.

Because en means in here, and the sentence is locating where the problem was.

• En la segunda columna = in the second column
• del formulario = of the form

So the full opening means In the second column of the form...

If you removed en, the sentence would no longer clearly express location.

Del is the contraction of de + el.

• de = of
• el = the
• de el becomes del

So:

• la segunda columna del formulario = the second column of the form

This contraction is mandatory in standard Spanish whenever de is followed by the masculine singular article el, unless El is part of a proper name.

For example:

• el título del libro
• la puerta del coche

But:

• de El Escorial (because El Escorial is a name)

Faltaba is the imperfect tense of faltar, while faltó is the preterite.

Here, faltaba is used because it describes a state or situation in the background: the surname was missing on the form.

• faltaba = was missing / was absent
• faltó = was missing at a specific moment / failed to be there

In this sentence, the missing surname is the situation that explains what happened next:

• ...así que tuve que... = ...so I had to...

So Spanish treats the missing information as an ongoing state, not as a single completed event.

Yes — that is exactly how Spanish often structures this idea.

The verb faltar commonly means:

• to be missing
• to be lacking
• to be absent

So:

• faltaba el apellido literally = the surname was missing

Spanish often makes the missing thing the subject of faltar.

Compare:

• Faltaba un dato. = A piece of information was missing.
• Faltan dos páginas. = Two pages are missing.

English and Spanish organize this idea similarly here, but learners sometimes expect a verb more like omitir or dejar fuera. Faltar is very natural.

Both are possible, but el apellido de la destinataria is more explicit.

• el apellido de la destinataria = the recipient’s surname
• su apellido = her surname

Spanish often prefers the more specific de + noun structure when there could be ambiguity. Since su can mean his, her, your, their, etc., de la destinataria makes it completely clear whose surname is meant.

It can also sound a bit more formal, which fits well with formulario.

Destinataria means female recipient or female addressee.

It is feminine because the person receiving the form, letter, or document is understood to be a woman.

• destinatario = male recipient / generic masculine form
• destinataria = female recipient

So:

• el apellido de la destinataria = the female recipient’s surname

Spanish often marks gender in nouns referring to people, especially in formal words like this.

Yes. Depending on context, destinataria can mean:

• recipient
• addressee

If the context is a letter, package, or form, either can work.
In this sentence, recipient sounds natural because it refers to the person whose surname should appear on the form.

Así que means so, therefore, or as a result.

It introduces the consequence:

• faltaba el apellido..., así que tuve que...
• the surname was missing..., so I had to...

By contrast, porque means because and introduces the cause.

Compare:

• Tuve que volver a escribir la página entera porque faltaba el apellido. = I had to rewrite the whole page because the surname was missing.

• Faltaba el apellido, así que tuve que volver a escribir la página entera. = The surname was missing, so I had to rewrite the whole page.

Same idea, different direction.

Tuve que is the preterite of tener que, which means to have to.

• tener que + infinitive = to have to do something
• tuve que volver a escribir = I had to write again / rewrite

Here:

• tuve = I had
• que = part of the tener que structure
• volver a escribir = to write again

The preterite is used because this was a completed action or necessity in the past.

Compare:

• tenía que = I had to / I was supposed to (background, repeated, ongoing, less bounded)
• tuve que = I had to (specific completed occasion)

Both can mean to write again or to rewrite, but volver a + infinitive is extremely common in everyday Spanish.

• volver a escribir = to write again
• reescribir = to rewrite

In many situations, volver a escribir sounds more natural and less formal or technical.

Here it suggests the speaker had to do the writing again from the beginning or repeat the act of writing.

It can suggest either, and in this context the difference is very small.

Literally:

• volver a escribir = to write again
• la página entera = the whole page

So the sentence means that the speaker had to write the entire page again, which in natural English is often simply rewrite the whole page.

The usual neutral order is:

• noun + adjective
• la página entera = the whole page

Placing entera after the noun is the normal, straightforward choice.

You can sometimes move adjectives before the noun for emphasis or style, but la página entera is the most natural form here.

Also note that entero/a often means whole:

• el día entero = the whole day
• la ciudad entera = the whole city
• la página entera = the whole page

Yes. Toda la página and la página entera are very close in meaning.

• toda la página = the whole page / all the page
• la página entera = the whole page / the entire page

In many contexts, they are interchangeable.

There can be a slight nuance:

• toda la página often focuses on all of it
• la página entera often emphasizes the page in its entirety

But in this sentence, either would sound natural.

Spanish uses definite articles more often than English does.

In this sentence:

• la segunda columna
• del formulario
• el apellido
• de la destinataria
• la página entera

Spanish often includes the article where English might omit it or use a possessive instead.

For example:

• el apellido de la destinataria
  literally: the surname of the recipient
  natural English: the recipient’s surname

This is very normal Spanish style, especially in formal or descriptive sentences.

Because Spanish usually omits subject pronouns when the verb form already makes the subject clear.

• tuve clearly means I had
• so (yo) tuve que = I had to

Spanish is a pro-drop language, which means subject pronouns are often left out unless they are needed for emphasis, contrast, or clarity.

So:

• tuve que volver a escribir... = perfectly natural
• yo tuve que volver a escribir... = possible, but more emphatic

No.

• formulario = form in the sense of a document with boxes or fields to fill in
• forma = shape, way, or sometimes form in other abstract senses

Here, formulario is the correct word because the sentence refers to an actual document with columns.

Because Spanish normally uses an ordinal number for this kind of meaning:

• la segunda columna = the second column

Using dos would not be the normal way to say this in standard Spanish.

Compare:

• la primera página = the first page
• la segunda columna = the second column
• el tercer apartado = the third section

Yes.

• apellido = surname / last name
• nombre = first name / given name
• nombre y apellidos = full name, depending on context more literally given name and surnames

For a learner of Spanish from Spain, it is also useful to know that Spanish naming customs often involve two surnames, even though this sentence just says el apellido.