Τα χαρτιά που χρειάζομαι για την αίτηση είναι ήδη στο ντοσιέ μου.

Yes. Τα χαρτιά literally means the papers, but in many real-life contexts it often means documents / paperwork.

So in this sentence, τα χαρτιά is very naturally understood as the documents needed for the application.

A few useful points:

• το χαρτί = the paper
• τα χαρτιά = the papers / documents

Greek often uses χαρτιά where English might prefer documents, especially in bureaucratic contexts.

Here που introduces a relative clause. It means that / which / that I need.

So:

• τα χαρτιά = the papers
• που χρειάζομαι = that I need

Together:

• τα χαρτιά που χρειάζομαι = the papers that I need

A very important point for learners: Greek που does not change form for gender, number, or case in this kind of sentence. It is much simpler than English relative pronouns like who/which/that.

Χρειάζομαι is the normal verb form meaning I need.

It comes from a verb that is used in the middle/passive-looking form, even though the meaning is active in English. This is very common in Greek.

So:

• χρειάζομαι = I need
• χρειάζεσαι = you need
• χρειάζεται = he/she/it needs

You should learn χρειάζομαι as the standard dictionary form for to need in everyday use.

In this sentence, χαρτιά is nominative plural, because it is the subject of είναι.

The full subject is actually:

• Τα χαρτιά που χρειάζομαι για την αίτηση

These are the things that are already in my folder.

You are right that for many neuter plural nouns, the nominative and accusative forms are the same:

• nominative plural: τα χαρτιά
• accusative plural: τα χαρτιά

So you identify the case by the role in the sentence, not just by the form.

Here για means for.

So:

• για την αίτηση = for the application

It explains what the papers are needed for.

Also, για takes the accusative case, which is why you get:

• η αίτηση = the application (nominative)
• την αίτηση = the application (accusative)

So after για, Greek uses την αίτηση, not η αίτηση.

Because αίτηση is a feminine singular noun, and after για it must be in the accusative singular.

The forms are:

• η αίτηση = nominative
• την αίτηση = accusative

Since για requires the accusative, Greek says:

• για την αίτηση

This is one of the most important article patterns to get used to in Greek.

Είναι ήδη means are already.

Greek word order is more flexible than English. So ήδη can often move around more easily than already in English.

In this sentence:

• είναι ήδη στο ντοσιέ μου = are already in my folder

This sounds completely natural.

You could also hear slightly different word orders in Greek depending on emphasis, but this version is standard and neutral.

Στο is a contraction of:

• σε + το = στο

Here σε means in / at / on, depending on context.

So:

• στο ντοσιέ = in the folder

This contraction is completely normal and standard in Greek.

Similar contractions include:

• στο = σε το
• στη = σε τη(ν)
• στον = σε τον

Ντοσιέ is a borrowed word, used in Greek for folder / file / binder, depending on context.

It is very common in modern Greek, especially in office or document-related contexts.

Its article is το, so it is treated as a neuter noun:

• το ντοσιέ = the folder
• στο ντοσιέ = in the folder

Many borrowed words in Greek are either indeclinable or only lightly adapted, and ντοσιέ is one of those words learners often meet early.

Because μου here is a weak possessive pronoun meaning my.

Greek very often expresses possession this way:

• το βιβλίο μου = my book
• ο φίλος μου = my friend
• το ντοσιέ μου = my folder

So instead of putting a separate word before the noun like English my, Greek commonly puts μου after the noun phrase.

That is the normal pattern.

Greek uses the definite article more regularly than English.

In this sentence, each noun is definite and specific:

• τα χαρτιά = the papers/documents
• την αίτηση = the application
• το ντοσιέ μου = my folder

In English, sometimes articles are dropped more easily, especially in phrases like application papers or in my folder. Greek is usually more explicit with articles, and learners need to get used to that.

Yes, absolutely. That is a very close, literal breakdown:

• Τα χαρτιά = the papers
• που χρειάζομαι = that I need
• για την αίτηση = for the application
• είναι ήδη = are already
• στο ντοσιέ μου = in my folder

That kind of word-by-word mapping is very useful for understanding the grammar.

It is neutral standard Greek. It sounds natural in everyday speech and also works perfectly well in a practical or semi-formal context, especially because the topic is paperwork.

Nothing in the sentence is especially slangy or especially formal. It is just normal modern Greek.

Yes. The full noun phrase τα χαρτιά που χρειάζομαι για την αίτηση is the subject.

A useful way to see it is this:

• Main clause: Τα χαρτιά ... είναι ήδη στο ντοσιέ μου
• Relative clause inside the subject: που χρειάζομαι για την αίτηση

So the main statement is about the papers:

• The papers are already in my folder

The relative clause simply adds information:

• which I need for the application

This is exactly like English structure:

• The papers that I need for the application are already in my folder.