Πόσα βιβλία έχεις στο σαλόνι;

Πόσα means how many.

It comes from the adjective πόσος, -η, -ο (how much / how many). Like other Greek adjectives, it changes form to agree with the noun in:

• gender
• number
• case

Here the noun is βιβλία (books), which is:

• neuter
• plural
• accusative

So πόσος must take the neuter plural accusative form:

• masculine plural: πόσοι
• feminine plural: πόσες
• neuter plural: πόσα

Because βιβλία is neuter plural, we say Πόσα βιβλία = How many books.

The singular is βιβλίο (book), a neuter noun ending in -ο.

Most modern Greek neuter nouns ending in -ο form the plural with -α:

• το βιβλίο → τα βιβλία (the book → the books)
• το σπίτι → τα σπίτια (the house → the houses)
• το παιδί → τα παιδιά (the child → the children)

So βιβλίο becomes βιβλία in the plural. There is no form βιβλίοι in modern Greek.

Έχεις means you have (informal, singular).

It is the 2nd person singular of the verb έχω (to have):

• έχω – I have
• έχεις – you have (singular, informal)
• έχει – he/she/it has
• έχουμε – we have
• έχετε – you have (plural or formal)
• έχουν(ε) – they have

So Πόσα βιβλία έχεις; = How many books do you have? (talking to one person you know well).

In Greek, the personal subject pronoun is often omitted, because the verb ending already shows the person:

• έχεις already tells us it is you (singular).

Saying εσύ έχεις is possible, but it adds emphasis:

• Εσύ πόσα βιβλία έχεις;
  = How many books do you have? (you, in contrast to someone else?)

In neutral, everyday speech, just Πόσα βιβλία έχεις; is normal.

Στο is a contraction of:

• σε (in, at, on, to)
• το (the – neuter singular article)

So:

• σε + το σαλόνι → στο σαλόνι = in the living room

Greek very often contracts σε with the article:

• σε + τον → στον (masc. sing.)
• σε + την → στη(ν) (fem. sing.)
• σε + το → στο (neut. sing.)
• σε + τους → στους (masc. plural)
• σε + τις / τας → στις (fem. plural)
• σε + τα → στα (neut. plural)

In Greek, the definite article is used much more often than in English.

• English: in the living room / in the living room (sometimes you might even say just in living rooms generally, but for your own house you usually keep the).
• Greek: talking about your living room, you normally say στο σαλόνι (literally in the living room).

Leaving out the article (just σε σαλόνι) would sound strange or very non‑standard in this context. The article is almost always present with common, specific places like το σπίτι, το σαλόνι, το σχολείο etc.

Both are in the accusative case:

• βιβλία – direct object of the verb έχεις
  
  • You have what? → books → accusative plural neuter.
• σαλόνι – object of the preposition σε (στο)
  
  • Preposition σε always takes the accusative.

So:

• τα βιβλία (nom./acc. plural neuter – same form) → here used as accusative
• το σαλόνι (nom./acc. singular neuter – same form) → here used as accusative after σε

Yes, you can change the word order. All of these are grammatically correct, with slightly different emphasis:

• Πόσα βιβλία έχεις στο σαλόνι;
  Neutral, focuses first on how many books.
• Στο σαλόνι πόσα βιβλία έχεις;
  Puts mild emphasis on in the living room, as if contrasting it with other rooms.
• Πόσα βιβλία στο σαλόνι έχεις;
  Also possible, with a slight stylistic shift; still understandable.

Greek word order is relatively flexible, but the given sentence is the most natural, default order in everyday speech.

In this sentence, έχεις clearly means you have.

Colloquially, Greek sometimes uses έχει in an impersonal way, almost like there is / there are, especially with weather or availability, but:

• έχεις specifically includes you.
• For there are… we more often use υπάρχουν (there exist).

So:

• Έχεις βιβλία στο σαλόνι; = Do you have books in the living room?
• Υπάρχουν βιβλία στο σαλόνι; = Are there books in the living room?

In Greek typography, the question mark is written with the character that looks like an English semicolon ;

So:

• English: How many books do you have in the living room?
• Greek: Πόσα βιβλία έχεις στο σαλόνι;

What looks like ; in Greek is actually the question mark; the English‑style ? is not used in standard Greek punctuation.

The accent mark in Greek (´) shows stress, not a change in word meaning (in modern Greek).

In Πόσα, the accent indicates that the stress falls on the first syllable:

• ΠΟ‑σα (not Πο‑ΣΑ)

If you write Ποσα without an accent, it is simply orthographically incorrect in standard modern Greek; the intended meaning is still clear to a human reader, but the stress mark is required in correct writing.

For formal singular or plural you, you use έχετε:

• Πόσα βιβλία έχετε στο σαλόνι;

This can mean:

• How many books do you have in the living room? (talking politely to one person, e.g. a stranger, a teacher)
• How many books do you (all) have in the living room? (talking to several people)

Context usually makes clear whether it is formal singular or plural. The rest of the sentence stays the same.