Para una ausencia justificada, debo adjuntar un justificante.

Here para means for in the sense of in the case of or when dealing with.

So Para una ausencia justificada means something like:

• For a justified absence
• In the case of a justified absence

It sets the context for the rest of the sentence.

It is a noun phrase:

• ausencia = absence
• justificada = justified

In Spanish, adjectives usually come after the noun, so:

• una ausencia justificada = a justified absence

Also, justificada is feminine singular because ausencia is feminine singular.

Compare:

• un documento firmado = a signed document
• una ausencia justificada = a justified absence

Spanish often uses an article where English might or might not.

Here una ausencia means an absence or a case of absence. It sounds natural in this kind of general administrative sentence.

If you removed una, the sentence would sound less natural in this context.

Debo is the first person singular of deber.

Here it means:

• I must
• I have to
• I am supposed to

So:

• debo adjuntar = I must attach

It is a little more formal than casual spoken English I’ve gotta attach.

Yes. Both are correct.

• debo adjuntar = I must attach
• tengo que adjuntar = I have to attach

In many situations, tengo que sounds a bit more common in everyday speech, while debo can sound slightly more formal or administrative. In a sentence like this, debo fits very well.

Because after modal-type verbs like deber, Spanish uses the infinitive directly.

So:

• debo adjuntar
• not debo de adjuntar in this meaning
• not debo a adjuntar

This works like:

• quiero comer = I want to eat
• puedo ir = I can go
• debo adjuntar = I must attach

English uses to in some patterns, but Spanish often does not.

Adjuntar means to attach, especially with documents, forms, emails, or files.

Examples:

• adjuntar un archivo = to attach a file
• adjuntar un documento = to attach a document

It is not exactly the same as añadir:

• adjuntar = attach, enclose
• añadir = add

In administrative or office Spanish, adjuntar is the natural verb here.

A justificante is a supporting document, proof, or official note that justifies something.

In this sentence, it means a document proving that the absence is valid or excused.

For example, a justificante could be:

• a doctor’s note
• an official certificate
• a signed document from an institution

In Spain, justificante is a very common word in school, work, and administrative contexts.

They are related, but they do different jobs in the sentence:

• justificada is an adjective: it describes the absence
  
  • una ausencia justificada = a justified absence
• justificante is a noun: it is the document that proves or justifies it
  
  • adjuntar un justificante = attach supporting proof

So one describes the absence, and the other names the proof.

Because the gender depends on each noun individually.

• ausencia is feminine → una ausencia
• justificante is masculine → un justificante

There is no need for them to match each other, because they are different nouns.

Yes, that is grammatically correct.

Both versions work:

• Para una ausencia justificada, debo adjuntar un justificante.
• Debo adjuntar un justificante para una ausencia justificada.

The original sentence puts the condition first, which sounds especially natural in instructions, rules, or formal notices.

It gives the feeling of:

• As for a justified absence...
• In the case of a justified absence...

Yes, it sounds fairly formal and administrative.

Words like these make it sound official:

• ausencia
• justificada
• debo
• adjuntar
• justificante

This is exactly the kind of Spanish you might see in:

• school regulations
• HR forms
• attendance policies
• official instructions

In casual conversation, people might say something simpler, such as:

• Si falto con motivo justificado, tengo que llevar un justificante.

But the original sentence is very natural in formal Spanish from Spain.

Yes, justificante is very common in Spain, especially in schools, workplaces, and official settings.

A learner of Peninsular Spanish should definitely know it.

Depending on the country, other words may also appear in similar contexts, but for Spain, justificante is perfectly standard and very useful.