Ο φάκελος δικαιολογητικών είναι απαραίτητος για την ηλεκτρονική υποβολή.

Φάκελος literally can mean envelope, but in administrative and official contexts it often means:

• file
• folder
• application packet
• dossier

So in this sentence, Ο φάκελος δικαιολογητικών does not usually mean a physical mailing envelope. It means the set/folder of required supporting documents.

Context is what tells you which English word fits best.

Δικαιολογητικών comes from the noun δικαιολογητικό, which means:

• supporting document
• justification document
• required document

In this sentence, δικαιολογητικών is in the genitive plural, meaning of supporting documents.

So:

• φάκελος = folder/file
• δικαιολογητικών = of supporting documents

Together: folder of supporting documents

This is a very common Greek pattern:
noun + genitive noun = noun of noun

For example:

• το βιβλίο του μαθητή = the student’s book / the book of the student
• φάκελος δικαιολογητικών = folder of supporting documents

Greek often leaves out the article in this kind of “noun + genitive” expression, especially in formal or administrative language.

So φάκελος δικαιολογητικών is perfectly natural and means:

• a folder of supporting documents
• the supporting-documents folder/file, depending on context

You could sometimes see an article in other contexts, but here the article is not needed.

Because φάκελος is a masculine noun.

Greek articles must match the noun’s gender, number, and case.

Here:

• ο = masculine singular nominative
• φάκελος = masculine singular nominative

So:

• ο φάκελος = the folder/file

If it were neuter, you would use το.
If it were feminine, you would use η.

Because it agrees with φάκελος.

The adjective απαραίτητος means necessary / required / essential, and Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Since φάκελος is masculine singular nominative, the adjective is also masculine singular nominative:

• ο φάκελος
• είναι απαραίτητος

If the noun were feminine, you would get απαραίτητη.
If it were neuter, απαραίτητο.

Because here it is a predicate adjective, not an adjective placed directly before or after the noun.

The structure is:

• Ο φάκελος δικαιολογητικών = the folder of supporting documents
• είναι = is
• απαραίτητος = necessary

So literally:

• The folder of supporting documents is necessary

This is different from a phrase like:

• ο απαραίτητος φάκελος = the necessary folder

In your sentence, the adjective comes after είναι because it is part of the predicate.

It is in the accusative singular because it follows the preposition για.

In Modern Greek, για is normally followed by the accusative.

So:

• για = for
• την ηλεκτρονική υποβολή = the electronic submission

More specifically:

• την = feminine singular accusative article
• ηλεκτρονική = feminine singular accusative adjective
• υποβολή = feminine singular accusative noun

Because υποβολή is a feminine singular noun, and both the article and adjective have to agree with it.

So we get:

• την = feminine singular accusative article
• ηλεκτρονική = feminine singular accusative adjective
• υποβολή = feminine singular accusative noun

All three match.

This kind of agreement is very important in Greek.

Υποβολή means submission.

In administrative or online-service language, it usually refers to:

• submitting an application
• submitting documents
• sending information through a platform/system

So ηλεκτρονική υποβολή means:

• electronic submission
• online submission

Here it clearly means electronic or online.

Greek usually distinguishes:

• ηλεκτρονικός / ηλεκτρονική / ηλεκτρονικό = electronic
• ηλεκτρικός / ηλεκτρική / ηλεκτρικό = electric

So:

• ηλεκτρονική υποβολή = electronic submission
• not electric submission

In Modern Greek, the verb είμαι (to be) is usually expressed in normal statements like this.

So:

• Ο φάκελος δικαιολογητικών είναι απαραίτητος = The folder of supporting documents is necessary

Unlike some languages, Greek does not normally drop είναι in standard sentences of this type.

In headlines or very compressed note-style language, verbs may sometimes be omitted, but in regular prose this sentence needs είναι.

Yes, Greek word order is more flexible than English, although not all versions sound equally neutral.

The given sentence:

• Ο φάκελος δικαιολογητικών είναι απαραίτητος για την ηλεκτρονική υποβολή.

is the most straightforward and natural.

You might also hear or see variations like:

• Για την ηλεκτρονική υποβολή, ο φάκελος δικαιολογητικών είναι απαραίτητος.

This puts extra emphasis on for the electronic submission.

But the original order is the most standard for a learner to remember.

Yes. It sounds formal, official, and bureaucratic.

Words like:

• δικαιολογητικά
• υποβολή
• απαραίτητος

are very common in government, university, application, and administrative contexts.

A more everyday way of saying something similar might be less formal, but this sentence is exactly the kind of Greek you would expect in instructions, forms, or official websites.

A very useful way is:

• Ο φάκελος δικαιολογητικών = the folder/file of supporting documents
• είναι απαραίτητος = is necessary / is required
• για την ηλεκτρονική υποβολή = for the electronic submission / for online submission

So the grammar becomes easier to see:

subject + verb + adjective + prepositional phrase

A rough pronunciation guide is:

• Ο φάκελος → o FA-ke-los
• δικαιολογητικών → di-ke-o-lo-yi-ti-KON
• είναι → EE-ne
• απαραίτητος → a-pa-RE-ti-tos
• για → ya
• την ηλεκτρονική → tin i-lek-tro-ni-KI
• υποβολή → i-po-vo-LI

The written accent marks show you where the stress falls, and they are very important in Greek pronunciation.