Escribo mi nacionalidad en el formulario.

Because Spanish often drops subject pronouns when they are not needed.

In Escribo mi nacionalidad en el formulario, the verb escribo already tells you the subject is I, because the -o ending is the first-person singular form.

So both of these are correct:

• Escribo mi nacionalidad en el formulario.
• Yo escribo mi nacionalidad en el formulario.

Adding yo is usually for emphasis, contrast, or clarity.

For example:

• Yo escribo mi nacionalidad, y tú escribes tu dirección.

Escribir is the infinitive, meaning to write.

In a full sentence like this, Spanish normally needs a conjugated verb, so escribir changes to escribo to match I in the present tense.

Conjugation of escribir in the present:

• yo escribo = I write
• tú escribes = you write
• él/ella escribe = he/she writes
• nosotros escribimos = we write
• ellos escriben = they write

So escribo means I write or sometimes I am writing, depending on context.

It can mean either one, depending on context.

Spanish simple present often covers both:

• I write
• I am writing

So Escribo mi nacionalidad en el formulario could mean:

• I write my nationality on the form
• I am writing my nationality on the form

If you specifically want to emphasize the ongoing action, Spanish can use the progressive:

• Estoy escribiendo mi nacionalidad en el formulario.
  = I am writing my nationality on the form.

But the simple present is very common and natural.

These are two different words:

• mi = my (possessive adjective)
• mí = me after a preposition

So here you need mi because it modifies nacionalidad:

• mi nacionalidad = my nationality

Examples with mí:

• para mí = for me
• sin mí = without me

A helpful contrast:

• mi nombre = my name
• para mí = for me

Because the sentence is talking about what you write down, not directly stating what you are.

• mi nacionalidad = my nationality
• soy estadounidense = I am American

If a form asks for a category like nationality, Spanish often uses the noun:

• Escribo mi nacionalidad en el formulario.

If you were answering out loud in conversation, you might say:

• Soy mexicano.
• Soy colombiana.
• Soy canadiense.

So the sentence uses a noun phrase, which fits the idea of filling out a form.

Because nacionalidad is a feminine noun in Spanish.

You can tell from:

• the article: la nacionalidad
• the ending: many nouns ending in -dad are feminine

Examples:

• la ciudad
• la universidad
• la posibilidad
• la nacionalidad

That is why you would say:

• mi nacionalidad
• la nacionalidad

But notice that mi does not change form here. It can go with both masculine and feminine singular nouns:

• mi formulario
• mi nacionalidad

Spanish often uses en where English uses on.

So:

• en el formulario literally looks like in the form
• but the natural English translation is often on the form

This is very common with documents, pages, lists, and similar things.

Examples:

• en la lista = on the list
• en la página = on the page
• en el formulario = on the form

So this is a good example of a phrase you should learn as natural Spanish, not translate word-for-word.

Yes. Formulario is a masculine noun, so it takes el in the singular:

• el formulario = the form

This fits the common pattern that many nouns ending in -o are masculine.

Examples:

• el libro
• el documento
• el formulario

Its plural is:

• los formularios

No. Spanish only makes mandatory contractions in two cases:

• a + el = al
• de + el = del

But en + el does not contract.

So these are correct:

• en el formulario
• en el libro
• en el coche

Not:

• *nel formulario

So en el stays as two words.

Yes. Spanish word order is fairly flexible, though the original sentence is the most neutral.

Neutral order:

• Escribo mi nacionalidad en el formulario.

You could also say:

• En el formulario escribo mi nacionalidad.

That version puts more focus on where you write it.

In many cases, both are correct, but the emphasis changes slightly.

No, but it is a very standard and widely understood word.

In Latin America, you may also hear regional alternatives such as:

• planilla
• formato
• sometimes forma, though that can also mean shape or way

Still, formulario is a safe, clear choice in most contexts, especially for official or written forms.

A simple approximate pronunciation for an English speaker is:

es-KREE-bo mee na-syo-na-lee-DAD en el for-moo-LA-ryo

A few useful pronunciation notes:

• Escribo: the b is softer than English b
• mi: sounds like mee
• nacionalidad: stress is on the last syllable, -dad
• formulario: in much of Latin America, the r is tapped lightly

A rough syllable breakdown:

• Es-cri-bo
• mi
• na-cio-na-li-dad
• en
• el
• for-mu-la-rio

Yes. That is grammatically correct and means I am writing my nationality on the form.

The difference is mainly one of focus:

• Escribo mi nacionalidad en el formulario
  = simple present; can mean I write or I am writing
• Estoy escribiendo mi nacionalidad en el formulario
  = clearly emphasizes the action is happening right now

In everyday Spanish, the simple present is often enough, so escribo sounds very natural.