Το μάγουλό μου είναι κόκκινο, γιατί έχει πολύ ήλιο έξω.

Greek usually uses the definite article with body parts and many possessed nouns, so το μάγουλό μου is the normal way to say my cheek.

• το = the for a neuter singular noun
• μάγουλο = cheek
• μου = my

So literally it is something like the cheek of me, but in natural English that is just my cheek.

Without το, it would sound less natural here.

This is about stress. The basic word is μάγουλο, but when the weak possessive pronoun μου is added after it, Greek often adds an extra written accent to show the pronunciation clearly:

• μάγουλο
• το μάγουλό μου

This is a normal spelling pattern with some noun + enclitic combinations. The extra accent helps preserve the original stress pattern.

So you should learn το μάγουλό μου as the standard written form.

μου is the weak genitive form of the pronoun meaning my / to me / for me, depending on context.

Here it shows possession:

• το μάγουλό μου = my cheek

This is very common in Greek. Instead of using a separate word like English my before the noun, Greek often puts μου after the noun:

• το βιβλίο μου = my book
• η φίλη μου = my friend
• το χέρι μου = my hand

Because adjectives in Greek must agree with the noun in gender, number, and case.

μάγουλο is neuter singular, so the adjective must also be neuter singular:

• masculine: κόκκινος
• feminine: κόκκινη
• neuter: κόκκινο

So:

• το μάγουλο είναι κόκκινο = correct

If the noun were feminine or masculine, the adjective would change:

• η μύτη είναι κόκκινη = the nose is red
• ο λαιμός είναι κόκκινος = the neck is red

είναι means is. It is the 3rd person singular form of είμαι = to be.

So:

• Το μάγουλό μου είναι κόκκινο = My cheek is red

This is a very direct Greek equivalent of the English structure subject + is + adjective.

Yes, γιατί can mean both why and because.

The meaning depends on the sentence:

• Γιατί είναι κόκκινο; = Why is it red?
• Είναι κόκκινο γιατί έχει πολύ ήλιο. = It is red because there is a lot of sun.

This is very common in Greek and usually causes no confusion because the sentence structure makes the meaning clear.

This is a very common Greek weather expression. Έχει can be used impersonally to talk about weather conditions.

So:

• έχει ήλιο = it’s sunny / there is sun
• έχει πολύ ήλιο = there is a lot of sun / it’s very sunny
• έχει ζέστη = it’s hot
• έχει κρύο = it’s cold

Even though έχει literally means has, in these expressions English usually translates it more naturally as it’s or there is.

Because ήλιο here is the object of έχει, so it is in the accusative case:

• nominative: ο ήλιος = the sun
• accusative: τον ήλιο

After πολύ, the noun also stays in the accusative here:

• έχει πολύ ήλιο = there is a lot of sun

Compare:

• Ο ήλιος είναι δυνατός. = The sun is strong.
  Here ήλιος is the subject, so it is nominative.
• Έχει πολύ ήλιο. = There is a lot of sun.
  Here ήλιο is treated as the object, so it is accusative.

έξω means outside. It tells you where the sunny conditions are.

So:

• έχει πολύ ήλιο = it’s very sunny
• έχει πολύ ήλιο έξω = it’s very sunny outside

In English, outside may feel a bit obvious, but in Greek it is perfectly natural to add it for emphasis or clarity.

Yes, it is understandable and basically natural. It means that the speaker’s cheek is red because it is very sunny outside.

A very slightly more natural word order in everyday Greek might be:

• Το μάγουλό μου είναι κόκκινο, γιατί έξω έχει πολύ ήλιο.

Greek word order is flexible, so both are fine. The difference is mostly about emphasis.

Greek is simply talking about one cheek here: το μάγουλό μου = my cheek.

If you wanted to say my cheeks are red, you would use the plural:

• Τα μάγουλά μου είναι κόκκινα.

Notice how everything changes to plural:

• το μάγουλο → τα μάγουλα
• το μάγουλό μου → τα μάγουλά μου
• κόκκινο → κόκκινα

So the original sentence is specifically about one cheek.