Η ποιότητα του φαγητού στο εστιατόριο δεν είναι τόσο καλή όσο στο σπίτι.

It is the Greek pattern for as ... as.

So:

• τόσο καλή όσο ... = as good as ...
• δεν είναι τόσο καλή όσο ... = is not as good as ...

Greek usually uses both parts together:

• τόσο = as / so
• όσο = as

This is a very common comparison pattern.

Because it agrees with η ποιότητα, which is a feminine singular noun.

In Greek, adjectives must match the noun they describe in:

• gender
• number
• case

So here:

• η ποιότητα = feminine singular
• καλή = feminine singular

Even though φαγητό is neuter, the adjective is not describing φαγητό. It is describing ποιότητα.

Because after ποιότητα, Greek uses the genitive to mean quality of something.

So:

• η ποιότητα = the quality
• του φαγητού = of the food

This is a very normal Greek structure:

• η τιμή του βιβλίου = the price of the book
• το χρώμα του αυτοκινήτου = the color of the car
• η ποιότητα του φαγητού = the quality of the food

Here:

• του = genitive singular article
• φαγητού = genitive singular of φαγητό

στο is the contracted form of σε + το.

So:

• σε το εστιατόριο becomes στο εστιατόριο
• σε το σπίτι becomes στο σπίτι

Greek commonly uses σε with the accusative for location, especially in everyday language.

So:

• στο εστιατόριο = at/in the restaurant
• στο σπίτι = at home / in the house

Also, both εστιατόριο and σπίτι are neuter nouns, and in the singular their accusative form looks the same as the nominative.

Because in Greek, δεν normally goes directly before the finite verb.

So:

• είναι = is
• δεν είναι = is not

This is the standard way to negate present-tense verbs.

Examples:

• δεν ξέρω = I do not know
• δεν θέλω = I do not want
• δεν είναι = it is not

Because Greek often leaves out repeated words when the meaning is obvious.

So όσο στο σπίτι is a shortened, natural way of saying something like:

• όσο είναι στο σπίτι
• or more loosely, as it is at home

The listener understands that the comparison is still about the quality of the food.

English does this too:

• It’s not as good as at home

So the Greek sentence is perfectly natural in this shorter form.

Here it means home in the sense of at home.

The phrase στο σπίτι is very common and often works like the English expression at home.

Depending on context, σπίτι can mean:

• house
• home

But in this sentence, όσο στο σπίτι clearly means as at home or as it is at home.

Yes, but the structure and nuance change.

This sentence says:

• δεν είναι τόσο καλή όσο ... = it is not as good as ...

A different way to express a similar idea would be:

• Η ποιότητα του φαγητού στο εστιατόριο είναι χειρότερη από του σπιτιού.
• or more naturally in some contexts, ... είναι χειρότερη από αυτή στο σπίτι.

That means:

• is worse than

So the general idea is similar, but the grammar is different:

• τόσο ... όσο = as ... as
• χειρότερη από = worse than

Greek uses the definite article much more often than English does.

In this sentence you have:

• Η ποιότητα = the quality
• του φαγητού = of the food
• στο εστιατόριο = in the restaurant
• στο σπίτι = at home / in the house

Even where English might be a bit lighter with articles, Greek often keeps them.

This is one of the most important habits for English speakers to get used to: Greek article use is broader and more regular.

Not completely. Greek word order is more flexible than English word order.

The version you have is neutral and natural:

• Η ποιότητα του φαγητού στο εστιατόριο δεν είναι τόσο καλή όσο στο σπίτι.

But you could move things around for emphasis, for example:

• Στο εστιατόριο, η ποιότητα του φαγητού δεν είναι τόσο καλή όσο στο σπίτι.

That puts more focus on at the restaurant.

Even though Greek allows flexibility, not every order sounds equally natural. The original sentence is a good standard version.

It is in the nominative singular.

That is because it is the subject of the verb είναι.

So the basic structure is:

• Η ποιότητα ... δεν είναι τόσο καλή ...
• The quality ... is not as good ...

The subject is η ποιότητα, and both the article η and the adjective καλή match it.

Because καλή describes ποιότητα, not φαγητό.

This is a very common point of confusion for learners.

The sentence is not saying:

• the food is good

It is saying:

• the quality is good

And ποιότητα is feminine, so the adjective must also be feminine:

• η ποιότητα είναι καλή

If the adjective described το φαγητό, then it would be neuter:

• Το φαγητό είναι καλό. = The food is good.