Si falta un dato, tendrás que rellenar la solicitud otra vez.

After si for real, possible future situations, Spanish normally uses the present indicative, not the future.

So:

• Si falta un dato... = If a piece of information is missing...

Then the main clause can use the future:

• ...tendrás que rellenar la solicitud otra vez.

This pattern is very common:

• Si vienes, te ayudo.
• Si llueve, nos quedamos en casa.
• Si falta un dato, tendrás que...

Using si faltará would not be correct here.

Here, falta means is missing.

So falta un dato literally means:

• a piece of information is missing

It comes from the verb faltar, which often means:

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

Examples:

• Falta una firma. = A signature is missing.
• Faltan dos páginas. = Two pages are missing.

Notice that the thing missing is the grammatical subject:

• Falta un dato = One piece of information is missing
• Faltan dos datos = Two pieces of information are missing

Not exactly. In this sentence, un dato means a piece of information or a detail that is required on a form.

In everyday Spanish, dato is often countable:

• un dato = one piece of information
• dos datos = two pieces of information

So here it does not sound technical in the English data sense. It is more like:

• a personal detail
• a required entry
• a bit of information on a form

For example, on an application form, datos personales means personal details/personal information.

Tendrás que means you will have to.

It is made up of:

• tendrás = future of tener (you will have)
• que
• infinitive: rellenar

So:

• tendrás que rellenar = you will have to fill in

This is a very common structure:

• tener que + infinitive = to have to do something

Examples:

• Tengo que estudiar. = I have to study.
• Tienes que esperar. = You have to wait.
• Tendrás que volver. = You will have to come back.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

Here:

• tendrás already tells you the subject is tú

So:

• tendrás que rellenar... = you will have to fill in...

You could say tú tendrás que..., but it is usually unnecessary unless you want emphasis or contrast.

For example:

• Tú tendrás que rellenarla, no yo. = You will have to fill it in, not me.

All three can sometimes translate as to fill in/complete, but there are differences.

• rellenar: very common in Spain for filling in a form
• llenar: more literally to fill
• completar: to complete

In Spain, with forms, rellenar is extremely natural:

• rellenar un formulario
• rellenar una solicitud

Completar can also work, especially if the idea is finishing all the required information.

Llenar is less idiomatic in Spain for paperwork, though native speakers would understand it.

So in this sentence, rellenar la solicitud sounds very natural for Spain Spanish.

It can correspond to either fill in or fill out in English, depending on your variety of English.

In this context, rellenar la solicitud means:

• to complete the application form by writing the required information

It does not mean refill in the English sense of filling something again with liquid or material.

So here:

• rellenar la solicitud otra vez = fill in/fill out the application again

La solicitud means the application or the application form, depending on context.

It comes from solicitud, which can mean:

• an application
• a request

In bureaucratic or official contexts, it often refers to a form you submit.

Related words:

• un formulario = a form
• una solicitud = an application / request

So rellenar la solicitud is very natural if you are talking about an application form for a job, permit, service, etc.

Yes, otra vez and de nuevo can both mean again here.

• otra vez = again / one more time
• de nuevo = again / anew

In this sentence, both are possible:

• ...rellenar la solicitud otra vez
• ...rellenar la solicitud de nuevo

Otra vez is very common and natural in everyday speech.

Sometimes de nuevo can sound slightly more formal or neutral, but in many contexts they are interchangeable.

Yes, and it is actually very natural.

Difference:

• Si falta un dato... = If one piece of information is missing...
• Si falta algún dato... = If any piece of information is missing...

Algún dato sounds a bit more general and is very common in instructions or warnings on forms.

So both are correct, but:

• un dato can feel a little more literal/specific
• algún dato often feels more like general advice

Because solicitud is a feminine noun in Spanish:

• la solicitud

There is no special reason you can always predict from English; noun gender has to be learned with the word.

A useful tip: many nouns ending in -tud are feminine:

• la solicitud
• la actitud
• la juventud

So when learning nouns, it is best to learn them with the article:

• la solicitud, not just solicitud

You would make dato plural and the verb would also change:

• Si faltan datos, tendrás que rellenar la solicitud otra vez.
• Si faltan dos datos, tendrás que rellenar la solicitud otra vez.

Why? Because the verb agrees with the thing that is missing:

• falta un dato
• faltan datos

This is a very useful pattern to remember with faltar.

Yes, especially because of rellenar.

In Spain, rellenar una solicitud sounds very standard and natural. In Latin America, people may also say it, but llenar or completar can be more common depending on the country.

Everything else in the sentence is widely understood across the Spanish-speaking world:

• Si falta un dato...
• tendrás que...
• la solicitud
• otra vez

So the sentence is correct generally, but rellenar is a particularly Spain-friendly choice.