Learn Hardcore Greek: Ο αδερφός μου είναι μικρότερος από εμένα.

The definite article Ο (masculine nominative singular) is required here; it sounds incomplete or wrong without it:
✗ Αδερφός μου είναι μικρότερος… (unnatural as a full sentence in modern Greek)

Why is μου after αδερφός and not before it, like μου αδερφός?

In modern Greek, possessive pronouns like μου (my), σου (your), του (his), etc. usually come after the noun:

ο αδερφός μου = my brother
το βιβλίο σου = your book
η φίλη του = his friend

Putting μου before the noun (μου αδερφός) is not standard; it sounds wrong in modern Greek.

You can, however, emphasize possession in another way:

ο δικός μου αδερφός = my brother (as opposed to someone else’s)

But the neutral, normal form is:

ο αδερφός μου

What is the difference between αδερφός and αδελφός? Which one should I use?

They are essentially the same word with two different spellings/pronunciations.

αδερφός is the more colloquial/modern everyday spelling and pronunciation.
αδελφός is a bit more formal/archaic and closer to the ancient spelling.

In speech, most people say something close to [aðerˈfos] (αδερφός).

For everyday modern Greek, you can safely use αδερφός.

Why is μικρότερος used instead of just μικρός? What does the -τερος ending do?

μικρός means “small / young.”

To make a comparative (“smaller / younger”), Greek often adds -τερος:

μικρός → μικρότερος = smaller / younger
μεγάλος → μεγαλύτερος = bigger / older
γρήγορος → γρηγορότερος = faster

So μικρότερος is the comparative form of μικρός.

In this sentence:

είναι μικρότερος από εμένα = is younger than me

Could I say πιο μικρός από εμένα instead of μικρότερος από εμένα? Is there any difference?

Yes, both are correct and very common:

Ο αδερφός μου είναι μικρότερος από εμένα.
Ο αδερφός μου είναι πιο μικρός από εμένα.

Both mean “My brother is younger than me.”

Notes:

μικρότερος = “younger/smaller” with the -τερος comparative ending.
πιο μικρός = “more small/young,” using πιο (more) + adjective.

In everyday speech, πιο μικρός is extremely common and often feels a bit more conversational, but μικρότερος is also completely natural. There is no real change in meaning here.

Why is από used for “than”? I thought από means “from.”

You’re right that από primarily means “from”, but in modern Greek it’s also used after comparatives to mean “than”:

είμαι ψηλότερος από εσένα = I am taller than you
είναι καλύτερη από μένα = she is better than me

So in:

μικρότερος από εμένα → younger than me

Greek doesn’t use a special word like English “than”; instead it reuses από in this comparative structure.

You may also see παρά used in comparisons:

μικρότερος παρά εμένα

This is also correct but sounds a bit more stylistic or emphatic; από is more neutral and common.

Why is it από εμένα and not από εγώ? What’s the role of εμένα here?

εγώ and εμένα are both forms of the pronoun “I / me”, but they are used in different roles:

εγώ = subject form (“I”)
εμένα = stressed object form (“me”)

After a preposition like από, Greek requires the object form:

από εμένα = from/than me
για εμένα = for me
σε εμένα = to me

So:

✗ από εγώ (incorrect)
✓ από εμένα (correct)

In everyday speech, people often shorten εμένα to μένα:

από μένα = also “than me”; very common and natural.

So both:

μικρότερος από εμένα
μικρότερος από μένα

are correct. από μένα sounds slightly more colloquial.

Why is μικρότερος in the masculine form? How would this change for a sister instead of a brother?

Adjectives in Greek must agree with the noun in:

gender (masculine / feminine / neuter)
number (singular / plural)
case

Here the noun is ο αδερφός (masculine, singular, nominative), so the adjective is:

μικρότερος (masculine, singular, nominative)

If you were talking about a sister (η αδερφή), you’d use the feminine form:

Η αδερφή μου είναι μικρότερη από εμένα.

Basic forms of this adjective:

Masculine: μικρότερος
Feminine: μικρότερη
Neuter: μικρότερο

Plural examples:

Οι αδερφοί μου είναι μικρότεροι από εμένα. (my brothers are younger than me)
Οι αδερφές μου είναι μικρότερες από εμένα. (my sisters are younger than me)

Could I leave out από εμένα and just say Ο αδερφός μου είναι μικρότερος?

Yes, grammatically you can say:

Ο αδερφός μου είναι μικρότερος.

By itself, this means “My brother is younger / smaller.”
But younger/smaller than whom or what? It’s incomplete unless the context makes it obvious:

If the previous sentence is “Είμαι τριάντα χρονών.” then
Ο αδερφός μου είναι μικρότερος. clearly means younger than me.

In the standalone sentence you gave, από εμένα is included to explicitly say “than me,” which makes it clear without extra context.

Is the word order fixed? Could I say Ο αδερφός μου είναι από εμένα μικρότερος?

Normal, neutral word order is:

Ο αδερφός μου είναι μικρότερος από εμένα.

Greek word order is fairly flexible, so:

Ο αδερφός μου είναι από εμένα μικρότερος.

is possible and understandable, but it sounds a bit marked or emphatic, as if you’re stressing the “than me” part.

For a learner, it’s best to stick to the standard, most natural order:

[subject] + είναι + [adjective] + από + [pronoun]
→ Ο αδερφός μου είναι μικρότερος από εμένα.

How do you pronounce the words in this sentence?

Approximate pronunciations (stressed syllable in bold):

Ο → /o/ (like “o” in “or,” short)
αδερφός → a‑ðer‑FÓS
- α = “a” in “father”
- δ = th in “this” (voiced)
- ος → “os”
μου → /mu/ (like “moo”)
είναι → Í‑ne (EE‑neh)
μικρότερος → mi‑KRÓ‑te‑ros
- ι = “ee”
- ο = “o” as in “not”
από → a‑PÓ
εμένα → e‑MÉ‑na

Full sentence:
Ο αδερφός μου είναι μικρότερος από εμένα.
→ /o aðerˈfos mu ˈine miˈkroteros apo eˈmena/