Tengo que rellenar este formulario antes de ir al ayuntamiento.

Tener que + infinitive is one of the most common ways to say have to / need to in Spanish.

• tengo = I have
• que = links the idea to the verb that follows
• rellenar = to fill in

So tengo que rellenar means I have to fill in.

Other examples:

• Tengo que estudiar. = I have to study.
• Tenemos que salir. = We have to leave.

This is extremely common in everyday Spanish.

Yes, you could. Debo rellenar este formulario is grammatically correct.

But there is a slight difference in feel:

• tener que + infinitive = the most neutral, everyday way to say have to
• deber + infinitive = can sound a bit more formal, stronger, or more like should / ought to / must, depending on context

In ordinary speech, tengo que is often the most natural choice.

All three can be possible, but they are not always used in exactly the same way.

In Spain, rellenar un formulario is very common and natural. It means to fill in a form.

• rellenar = to fill in, especially forms, boxes, spaces, etc.
• llenar = to fill; often more general, like filling a glass or a room
• completar = to complete; also possible with forms, especially if you want to emphasize finishing all the required information

So in Spain:

• rellenar un formulario sounds very natural
• completar un formulario is also possible
• llenar un formulario may be understood, but rellenar is usually the more idiomatic choice here

Because formulario is a masculine noun.

In Spanish, demonstratives must agree with the noun:

• este for masculine singular
• esta for feminine singular

So:

• este formulario
• esta solicitud
• este documento
• esta oficina

A useful tip: nouns ending in -o are often masculine, though not always.

Spanish does not always match English word-for-word.

English says fill in a form, but Spanish usually just says:

• rellenar un formulario

The idea of in is already built into the verb choice and the expression. You do not need a separate preposition to mirror the English phrasing.

This is very common when comparing English and Spanish: the natural expression is often different even if the meaning is the same.

Because in Spanish, when antes is followed by an infinitive, you normally use:

antes de + infinitive

So:

• antes de ir = before going / before I go
• antes de comer = before eating
• antes de salir = before leaving

You should not normally say antes ir.

If a full clause follows, then the structure changes:

• antes de que vaya = before I go

But with an infinitive, it is:

• antes de ir

Because after antes de, Spanish often uses the infinitive when the subject is understood and does not need to be stated separately.

Here, the sentence means:

• I have to fill in this form before going to the town hall or
• before I go to the town hall

Spanish expresses that with:

• antes de ir al ayuntamiento

You would use a conjugated verb after antes de que:

• Tengo que rellenar este formulario antes de que vaya al ayuntamiento.

That version is possible, but it is longer and less natural here because the subject is clearly the same person.

Because a + el contracts to al in Spanish.

So:

• a + el = al

Examples:

• Voy al banco.
• Vamos al centro.
• Tengo que ir al ayuntamiento.

This contraction is basically mandatory, except with the article in certain fixed names or titles.

So a el ayuntamiento would be incorrect here.

In Spain, ayuntamiento usually refers to the town hall or local city council.

Depending on context, it can mean:

• the building where local government offices are
• the municipal government itself

In this sentence, it most likely refers to the place you go for local administrative tasks, such as forms, registration, documents, etc.

This is a very useful word in Spain because it comes up a lot in everyday bureaucracy.

Because Spanish often drops subject pronouns when they are clear from the verb form.

• tengo already tells you the subject is I
• so yo tengo is usually unnecessary

Spanish is a pro-drop language, which means subject pronouns are often omitted.

You could say Yo tengo que rellenar este formulario, but that would usually add emphasis, contrast, or clarity:

• Yo tengo que rellenarlo, no tú.

In a normal sentence, Tengo que... is more natural.

Spanish word order is somewhat flexible, but this version is the most natural and straightforward:

Tengo que rellenar este formulario antes de ir al ayuntamiento.

You can move parts of the sentence for emphasis, but some versions sound less neutral. For example:

• Antes de ir al ayuntamiento, tengo que rellenar este formulario.

That is also very natural and puts the time sequence first.

A version like:

• Tengo que rellenar antes de ir al ayuntamiento este formulario is grammatically possible, but it sounds less natural in ordinary conversation.

So the original order is a very good standard model.

Yes, but the meaning changes slightly.

• este formulario = this form
  You are pointing to a specific form that is near you, present in the situation, or already identified.
• el formulario = the form
  It refers to a specific form already known from context, but without the same this feeling.

So both can work, but este is more specific and immediate.

In most of Spain, rellenar is pronounced approximately like:

reh-yeh-NAR

A few useful points:

• rr is a strong rolled or trilled r
• ll in most of Spain sounds like y
• the stress falls on the last syllable: -nar

So:

• re- = short reh
• lle- = roughly yeh
• -nar = stressed NAR

Pronunciation varies across the Spanish-speaking world, but this is a good general guide for Spain.

Yes. It sounds natural and idiomatic in Spain.

Especially natural features for Spain include:

• rellenar un formulario
• ir al ayuntamiento

It sounds like something someone would say when dealing with paperwork or local administration.

A native speaker in Spain would understand it immediately, and it does not sound awkward or overly textbook-like.