Learn Hardcore Greek: Όλο το υλικό του μαθήματος, το οποίο στέλνεται με email, είναι πολύ καλά οργανωμένο. - All the course material, which is sent by email, is very well organized.

Υλικό is a neuter, uncountable noun here (like information, material in English).
The definite article and the adjective must agree with it in gender, number, and case:
- Noun: (το) υλικό – neuter, singular, nominative
- Article: το – neuter, singular, nominative
- Adjective όλος (all) in neuter singular nominative is όλο.

So you get: όλο (adj.) + το (article) + υλικό (noun).

You would not say όλος ο υλικός, because:

υλικός is an adjective (meaning material, physical), not the noun υλικό.
You need the noun υλικό (material), not the adjective.

What exactly does του μαθήματος mean and why is it in that form?

Του μαθήματος means of the lesson / of the course and shows possession or association: the material of the course.

Grammar:

μάθημα = lesson / class / course (neuter noun)
Nominative singular: το μάθημα
Genitive singular: του μαθήματος

The genitive (του μαθήματος) is used here to mean the course’s material, exactly like course material in English, but literally material of the course.

So το υλικό του μαθήματος = the material of the course / the course material.

What is the nuance of υλικό here? Does it mean “material” in the teaching sense?

Yes. Υλικό is a very common word in educational contexts and here it means teaching/learning material: notes, PDFs, slides, exercises, etc.

Some typical uses:

εκπαιδευτικό υλικό – educational material
διαφημιστικό υλικό – promotional material
έντυπο υλικό – printed material

If you said υλικά (plural), that would more often be materials as separate countable items (e.g. construction materials, cooking ingredients).
In this sentence, υλικό is uncountable, like English material in course material.

Why does the sentence use το οποίο instead of που? Are both possible?

Both are possible, but they differ in style and structure.

Το οποίο is a relative pronoun (neuter singular) that agrees with υλικό. It is more formal and explicit.
Που is an invariable relative particle; it does not change form and is more neutral/colloquial.

In this sentence:

το οποίο στέλνεται με email = which is sent by email (more formal, written style)
που στέλνεται με email = that is sent by email (very common in speech and informal writing)

So you could also say:

Όλο το υλικό του μαθήματος, που στέλνεται με email, είναι πολύ καλά οργανωμένο.

The meaning is the same; το οποίο simply sounds a bit more formal or “bookish”.

Why is it το οποίο (neuter) and not ο οποίος or η οποία?

The relative pronoun must agree in gender and number with the noun it refers to.

The noun is το υλικό – neuter, singular.
Therefore the relative pronoun is το οποίο – neuter, singular.

If the noun were masculine or feminine, you would see:

Masculine: ο μαθητής, ο οποίος… – the student, who…
Feminine: η δασκάλα, η οποία… – the teacher, who…

Here it’s το υλικό … το οποίο…, both neuter singular.

How does στέλνεται work grammatically? What tense and voice is it?

Στέλνεται is:

Present tense
Passive voice
3rd person singular

It comes from the active verb στέλνω (I send).

Conjugation pattern (present simple):

Active:
- εγώ στέλνω – I send
- εσύ στέλνεις – you send
- αυτός/αυτή/αυτό στέλνει – he/she/it sends
Passive:
- εγώ στέλνομαι – I am sent
- εσύ στέλνεσαι – you are sent
- αυτός/αυτή/αυτό στέλνεται – he/she/it is sent

So στέλνεται με email = is sent by email.
The subject is όλο το υλικό (all the material), so the verb is 3rd person singular.

Why is it με email? Could you say something else, like μέσω email?

Με email literally means with email / by email and is very commonly used in everyday Greek.

με here expresses means: by what means something is done.
So στέλνεται με email = it is sent by email.

Other natural options:

μέσω email – through email (a bit more formal)
στέλνεται με μήνυμα ηλεκτρονικού ταχυδρομείου – sent by electronic mail (very formal / written)

In normal conversation or casual writing, με email is perfectly idiomatic and often preferred.

Why is the relative clause between commas: …, το οποίο στέλνεται με email, …?

The commas show that το οποίο στέλνεται με email is non‑restrictive (extra information), not essential to identify the material.

Without commas, it would mean “the course material that is sent by email” (defining which material).
With commas, it means “the course material, which (by the way) is sent by email, …” – it’s additional information about that already-identified material.

In spoken Greek, the pause and intonation match those commas. In written Greek, it clarifies that this is a side comment about how it is delivered.

Why is it πολύ καλά οργανωμένο and not πολύ καλή οργανωμένο or something similar?

There are two points here:

Καλά vs καλή
- καλά is the adverb: well.
- καλός / καλή / καλό are the adjective forms: good.
- In πολύ καλά οργανωμένο, καλά modifies the past participle οργανωμένο like an adverb: very well organized, not very good organized.
Gender agreement of οργανωμένο
- υλικό is neuter singular → any adjective/participle describing it must also be neuter singular.
- οργανωμένος (m.), οργανωμένη (f.), οργανωμένο (n.)
- So you need οργανωμένο to agree with το υλικό.

Put together:

είναι πολύ καλά οργανωμένο = it is very well organized (neuter subject).

Why is οργανωμένο singular if όλο το υλικό might contain many items?

Grammatically, υλικό is treated as a single, uncountable mass noun (like material, stuff, information in English).

The subject is το υλικό – one neuter singular noun.
Therefore, the verb είναι and the participle οργανωμένο are also in the singular.

Even though in reality this material consists of many files or documents, Greek (like English) treats material as a singular mass:

English: The material is well organized.
Greek: Το υλικό είναι καλά οργανωμένο.

Could I write the sentence without commas, or change the word order, and still be correct?

You could change it a bit and still be correct, but the commas and order slightly change the nuance.

Without commas (more “defining”):
- Όλο το υλικό του μαθήματος το οποίο στέλνεται με email είναι πολύ καλά οργανωμένο.
  Now it can sound more like “All the course material that is sent by email is very well organized”, as if there might also be some material not sent by email.
Splitting into two clauses:
- Όλο το υλικό του μαθήματος στέλνεται με email και είναι πολύ καλά οργανωμένο.
  This is simpler: All the course material is sent by email and is very well organized.
  The information is the same, just expressed with coordination (και) instead of a relative clause.

The original version with commas and το οποίο is slightly more formal and “written style”, but all of these are grammatically correct.