Learn Hardcore Greek: Ο σκύλος της είναι λίγο παχύς, ενώ η γάτα της είναι πολύ λεπτή. - Her dog is a bit fat, while her cat is very thin.

It appears twice simply because Greek normally marks possession with each noun separately. So instead of saying “her dog and cat” with her only once, Greek more often says ο σκύλος της και η γάτα της (“the dog of hers and the cat of hers” literally).

You can sometimes drop the second της if the context is clear, but in careful or neutral speech, using it with each noun is very common and sounds natural.

Why is it Ο σκύλος but η γάτα? Why do the articles change?

The articles change because nouns in Greek have grammatical gender.

ο is the masculine definite article (singular, nominative)
η is the feminine definite article (singular, nominative)

In this sentence:

ο σκύλος = the dog (masculine noun)
η γάτα = the cat (feminine noun)

So you must match the article to the grammatical gender of each noun:

masculine: ο σκύλος, ο άντρας, ο φίλος
feminine: η γάτα, η γυναίκα, η φίλη
neuter: το παιδί, το βιβλίο, το σκυλί

Why is it παχύς for the dog and λεπτή for the cat? Why don’t the adjectives have the same form?

In Greek, adjectives agree with the noun they describe in:

gender
number
case

So:

ο σκύλος is masculine, singular, nominative, so the adjective must also be masculine, singular, nominative:
- λίγο παχύς (not παχύ)
η γάτα is feminine, singular, nominative, so the adjective must be feminine, singular, nominative:
- πολύ λεπτή (not λεπτός or λεπτό)

Rough patterns:

masculine often ends in -ος, -ής, -ύς → παχύς
feminine often ends in -η, -α → λεπτή
neuter often ends in -ο, -ι → παχύ, λεπτό

Even when the adjective comes after the verb είναι (“is”), it still agrees with the noun: ο σκύλος είναι παχύς, η γάτα είναι λεπτή.

Could I say Ο σκύλος της είναι λίγο παχύ instead of παχύς?

No, not when you’re talking about ο σκύλος (masculine).

Παχύ is the neuter form, but the noun ο σκύλος is masculine, so the adjective must also be masculine: παχύς.

Correct: Ο σκύλος της είναι λίγο παχύς.
Incorrect (for this meaning): Ο σκύλος της είναι λίγο παχύ.

You would use παχύ with a neuter noun, for example:

Το σκυλί της είναι λίγο παχύ. (“Her doggie / dog (neuter) is a bit fat.”)

So:

ο σκύλος → παχύς
το σκυλί → παχύ

What exactly does λίγο mean here? Is it “a bit”, “a little”, or “slightly”?

Λίγο literally means “a little / a bit”, and in this context it works as a degree adverb modifying the adjective παχύς:

είναι λίγο παχύς ≈ “he is a bit fat / slightly fat”

Common English equivalents, depending on tone:

a bit fat
a little fat
kind of fat
slightly overweight

Putting λίγο before the adjective tends to soften it, making the description sound less harsh than simply saying είναι παχύς (“he is fat”).

What about πολύ? Does it mean both “very” and “a lot”?

Yes. Πολύ can mean both “very” and “a lot / much / many”, depending on how it is used.

In your sentence, it is an adverb modifying an adjective:

η γάτα της είναι πολύ λεπτή = “her cat is very thin / really thin”

Other uses:

With adjectives/adverbs → “very”
- πολύ καλός = very good
- τρέχει πολύ = he runs a lot / very fast (depending on context)
With verbs or nouns, especially in the neuter form πολύ:
- τρώνε πολύ = they eat a lot
- έχει πολύ φαγητό = there is a lot of food

So here you should understand it as “very”: very thin.

What does ενώ mean, and what does it do in this sentence?

Ενώ means roughly “while”, “whereas”, or “whereas on the other hand”.

It is a conjunction that:

connects two clauses,
often shows contrast between them.

In your sentence:

Ο σκύλος της είναι λίγο παχύς, ενώ η γάτα της είναι πολύ λεπτή.
- “Her dog is a bit fat, whereas her cat is very thin.”

So ενώ sets up the contrast:

one animal is a bit fat,
the other is very thin.

You can also use ενώ in non-contrasting, purely temporal senses (“while”), but here it clearly introduces a comparison/contrast.

Is παχύς the usual word for “fat”? Does it sound rude?

For describing body fat, Greek more commonly uses χοντρός in everyday speech, especially for people, and also for animals:

Ο σκύλος της είναι λίγο χοντρός. = Her dog is a bit fat.

Παχύς is less frequent in everyday talk for people; you’ll see it:

in more formal or written language,
in fixed expressions (παχύ αίμα = thick blood),
in some dialects or older usage.

Nuance:

χοντρός: common, neutral in many contexts but can be rude depending on tone, just like “fat” in English.
παχουλός / παχουλή: a softer, more affectionate way to say “chubby”.

So a very natural, tactful sentence could be:

Ο σκύλος της είναι λίγο παχουλός, ενώ η γάτα της είναι πολύ λεπτή.
- “Her dog is a bit chubby, whereas her cat is very thin.”

Why is είναι used twice? Could you drop the second είναι?

The full sentence has είναι (“is”) in both clauses:

Ο σκύλος της είναι λίγο παχύς, ενώ η γάτα της είναι πολύ λεπτή.

In careful, neutral Greek this is perfectly normal and often preferred.

In informal speech, people sometimes omit the second είναι if the structure is obvious:

Ο σκύλος της είναι λίγο παχύς, ενώ η γάτα της πολύ λεπτή.
(Second είναι is understood.)

It’s still clear and grammatical, but beginners are usually advised to keep the verb in both clauses until they feel comfortable with omissions in natural speech patterns.

Why are σκύλος and γάτα in this particular form? Why not σκύλο or γάτας?

The forms σκύλος and γάτα here are in the nominative singular, because:

they are the subjects of the verbs είναι.

Greek uses different endings for different cases:

nominative: typically used for the subject of the sentence.
accusative: typically used for the object of a verb.
genitive: often used for possession, etc.

Examples:

Ο σκύλος της είναι λίγο παχύς.
- ο σκύλος: nominative (subject) → “the dog”
If it were an object:
- Βλέπω τον σκύλο της. = I see her dog.
  - τον σκύλο: accusative (object)

So here, σκύλος and γάτα are in the nominative because they are doing the “being”: the dog is a bit fat, the cat is very thin.

Can I say Το σκυλί της instead of Ο σκύλος της? Is there a difference?

Yes, you can say Το σκυλί της, and it’s very common.

Difference:

ο σκύλος (masculine): “the dog”
το σκυλί (neuter): also “the dog”, literally “little dog / doggie” but in modern Greek it’s often just the normal, neutral word for “dog,” especially in speech.

So:

Ο σκύλος της είναι λίγο παχύς.
Το σκυλί της είναι λίγο παχύ.

Both mean “Her dog is a bit fat.”
The only changes:

gender of the noun (masc → neuter),
corresponding adjective form (παχύς → παχύ),
article (ο → το).

Is the word order fixed, or could I say Η γάτα της είναι πολύ λεπτή, ενώ ο σκύλος της είναι λίγο παχύς?

You can absolutely reverse the order:

Η γάτα της είναι πολύ λεπτή, ενώ ο σκύλος της είναι λίγο παχύς.

This is grammatically correct and means the same thing. The difference is what you mention first and thus what you mentally highlight.

Greek word order is relatively flexible, especially within a simple sentence like this. Another possible (more marked/emphatic) order is:

Λίγο παχύς είναι ο σκύλος της, ενώ η γάτα της είναι πολύ λεπτή.
Ο σκύλος της είναι λίγο παχύς, ενώ πολύ λεπτή είναι η γάτα της.

These versions put extra emphasis on λίγο παχύς or πολύ λεπτή respectively. For learners, the straightforward word order in the original sentence is the best default.