En la lista falta mi segundo apellido, así que tendré que rellenar otra vez el formulario.

Because the thing that is missing is singular: mi segundo apellido.

In En la lista falta mi segundo apellido, the grammatical subject comes after the verb:

• falta = is missing
• mi segundo apellido = the thing that is missing

So Spanish agrees the verb with mi segundo apellido, not with la lista.

Compare:

• Falta mi segundo apellido. = My second surname is missing.
• Faltan mis dos apellidos. = My two surnames are missing.

Because en la lista means on the list or in the list, and it tells you where the omission is.

• En la lista = on the list / in the list
• falta mi segundo apellido = my second surname is missing

So the structure is basically:

• On the list, my second surname is missing.

If you said La lista falta..., that would sound like the list is missing..., which changes the meaning completely.

In Spain, people commonly have two surnames:

1. the first surname, usually from the father
2. the second surname, usually from the mother

So mi segundo apellido means my second family name/surname.

This is a very common cultural point in Spanish from Spain. In many official forms, both surnames are expected, so if one is missing, the form may be incomplete.

Because apellido means surname / last name, while nombre means first name / given name.

So:

• nombre = first name
• apellido = surname
• segundo apellido = second surname

A learner of Spanish from an English-speaking background may need to be careful here, because English naming customs are usually different.

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

It connects the first idea to the consequence:

• En la lista falta mi segundo apellido = My second surname is missing from the list
• así que tendré que rellenar otra vez el formulario = so I’ll have to fill out the form again

It is a very common connector in everyday Spanish.

You can think of it as introducing a logical result:

• problem → así que → consequence

Tendré is the future tense of tener:

• tendré = I will have

When tener que + infinitive is used, it means to have to do something.

So:

• tendré que rellenar = I will have to fill out

Breakdown:

• tener que = to have to
• tendré = I will have
• rellenar = fill out

This is a very common way to express obligation in Spanish.

Yes. Both are natural, but there is a slight difference in feel.

• Tendré que rellenar otra vez el formulario = I will have to fill out the form again
• Voy a tener que rellenar otra vez el formulario = I’m going to have to fill out the form again

In many everyday situations, they are almost interchangeable.
Very roughly:

• tendré que can sound a bit more neutral or formal
• voy a tener que can sound a bit more immediate or conversational

Both would be perfectly normal in Spain.

In Spain, rellenar un formulario is very common and often the most natural choice for to fill out a form.

• rellenar = fill in / fill out
• llenar = fill

With documents, forms, applications, etc., rellenar is especially common in Spain:

• rellenar un formulario
• rellenar una solicitud

Llenar is also understood, but in Spain rellenar is often preferred for paperwork.

Yes, in this sentence they are basically interchangeable.

• otra vez = again
• de nuevo = again / anew

So you could say:

• tendré que rellenar otra vez el formulario
• tendré que rellenar de nuevo el formulario

In everyday speech, otra vez is extremely common and sounds very natural.

Because it refers to a specific form that the speaker already has in mind.

• el formulario = the form
• un formulario = a form

Here, the idea is probably: there is a particular official form or document, and because the surname is missing, the speaker will have to fill out that same form again.

Using el makes it definite and specific.

Yes, somewhat. Spanish often allows more flexibility than English.

The given version:

• En la lista falta mi segundo apellido, así que tendré que rellenar otra vez el formulario.

Other possible versions:

• Falta mi segundo apellido en la lista, así que tendré que rellenar otra vez el formulario.
• Mi segundo apellido falta en la lista, así que tendré que rellenar otra vez el formulario.

However, the original sounds very natural because En la lista sets the context first, and falta mi segundo apellido presents the missing item afterward.

Sometimes, depending on context, but not always.

Formulario usually means form. In some contexts, especially administrative ones, that form may be part of an application process, but the word itself is still formulario = form.

If you specifically mean application as in an application for a job / grant / course, Spanish might use:

• solicitud
• instancia in some formal contexts
• sometimes formulario de solicitud = application form

So in this sentence, formulario is best understood simply as form.

Yes, very much so.

A few things make it feel especially natural in Spain:

• the cultural reference to segundo apellido
• the common use of rellenar un formulario
• the overall administrative/register style

A speaker from Latin America would understand it perfectly, but depending on the country, they might prefer slightly different wording, especially with document-related verbs.