Mi nacionalidad está en el formulario.

Because Spanish uses estar for location.

In this sentence, mi nacionalidad is being located in the form, so está is the natural choice.

A useful rule:

• ser = identity, definition, what something is
• estar = location, condition, state

So:

• Mi nacionalidad es española = this tells you what the nationality is
• Mi nacionalidad está en el formulario = this tells you where that information appears

Mi is the possessive adjective meaning my. It goes directly before the noun:

• mi nacionalidad = my nationality

That is the normal way to express possession here. Spanish usually does not say something like yo nacionalidad. The possessive word is what shows ownership.

In Spanish, a possessive adjective like mi, tu, su, nuestro, etc. usually replaces the article.

So Spanish says:

• mi nacionalidad not
• la mi nacionalidad in normal modern usage

This is the same pattern as:

• mi casa
• tu nombre
• su pasaporte

Because nouns ending in -dad are almost always feminine in Spanish.

So:

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

Even though the sentence uses mi, which does not change for masculine/feminine singular, the noun itself is feminine.

Because formulario is a masculine singular noun, so it takes el.

• el formulario = the form

Many nouns ending in -o are masculine, and formulario follows that common pattern.

Spanish often uses en where English might use in or on.

With documents, pages, forms, lists, and similar things, en is very common:

• en el formulario
• en la lista
• en la página
• en el documento

So even if English prefers on the form, Spanish naturally says en el formulario.

The accent mark is necessary because está is the he/she/it or you (formal) form of estar in the present tense.

• está = is
• esta = this (feminine), as in esta casa

So the accent helps distinguish different words.

Yes, but the given order is the most neutral and natural for a simple statement:

• Mi nacionalidad está en el formulario.

Spanish can change word order for emphasis or in questions, but that is less neutral. For example:

• ¿Está mi nacionalidad en el formulario? = a question
• En el formulario está mi nacionalidad = more emphasis on the location

For a plain statement, the original order is best.

Yes, but it changes the meaning slightly.

• Mi nacionalidad está en el formulario = specifically my nationality
• La nacionalidad está en el formulario = the nationality is on the form / nationality is on the form

So if you want to refer to your own nationality, mi is important.

Not always exactly.

Nacionalidad usually means nationality, and in many everyday situations that is the right word for forms and personal information.

But depending on the legal context, Spanish may also use:

• ciudadanía = citizenship

On many forms, though, nacionalidad is the normal label you will see.

It is the present tense of estar, third person singular.

The full present tense of estar is:

• yo estoy
• tú estás
• él/ella/usted está
• nosotros estamos
• vosotros estáis
• ellos/ellas/ustedes están

Here, the subject is mi nacionalidad, which is singular, so Spanish uses está.

You would usually just change the intonation in speech, and in writing add question marks:

• ¿Mi nacionalidad está en el formulario?

You can also invert the order:

• ¿Está mi nacionalidad en el formulario?

Both are possible, but ¿Está mi nacionalidad en el formulario? often sounds a bit more natural as a direct yes/no question.