Learn Hardcore Hungarian: A könyvtárban tilos enni. - Eating is forbidden in the library.

Hungarian suffixes obey vowel harmony. Words with any back vowel (a, á, o, ó, u, ú) take the back form -ban; words with only front vowels (e, é, i, í, ö, ő, ü, ű) take -ben. Könyvtár contains á (a back vowel), so it takes -ban: könyvtárban.
Examples:

kertben (“in the garden”) – only front vowels, so -ben
szobában (“in the room”) – has back vowel o, so -ban

What case is könyvtárban, and what are the related “in/into/out of” forms?

It’s the inessive case (“in”). The common interior-location trio is:

könyvtárban = in the library (inessive, -ban/-ben)
könyvtárba = into the library (illative, -ba/-be)
könyvtárból = out of/from the library (elative, -ból/-ből)

Why is there no word for “is” (no van)?

Hungarian drops the present-tense copula van when the predicate is a noun or an adjective. Tilos is an adjective (“forbidden”), so you say A könyvtárban tilos enni, not “A könyvtárban van tilos enni.”

What kind of word is tilos, and how is it used?

Tilos is an adjective meaning “forbidden/prohibited.” Common patterns:

With an infinitive: Tilos enni. (“Eating is forbidden.”)
With a noun/nominalization: Tilos az evés. (“Eating is forbidden.”)
Optionally specify who it’s forbidden for with the dative: A diákoknak tilos enni. (“For students, eating is forbidden.”)

How is tilos different from nem szabad or nem lehet?

tilos = expressly forbidden (strong, official/prohibitive; typical on signs)
nem szabad = not allowed (neutral, permission-focused)
nem lehet = not possible/you can’t (often used to mean “not allowed,” but literally about possibility)

All are used, but tilos sounds the strictest.

Can I change the word order? For example: Enni tilos a könyvtárban or A könyvtárban enni tilos?

Yes. Hungarian word order is flexible and reflects emphasis:

A könyvtárban tilos enni. – neutral, foregrounds the place.
Enni tilos a könyvtárban. – emphasizes that eating (as opposed to e.g. drinking) is forbidden.
A könyvtárban enni tilos. – slightly emphasizes the activity within that place.

All three are natural.

Why is the article A and not Az?

Use Az before vowel sounds, A before consonants. Könyvtár starts with consonant k, so A könyvtárban. Example with Az: Az iskolában (“in the school”), because iskola starts with a vowel.

Can I drop the article and say Könyvtárban tilos enni?

Yes. Without the article, it leans more generic/institutional (“At/Inside a library, eating is forbidden” or “In libraries in general…”). With A, it sounds more like a particular library.

How do I specify who it’s forbidden for?

Use the dative -nak/-nek, optionally with a personal infinitive:

Group: A diákoknak tilos enni a könyvtárban. (“Students are forbidden to eat in the library.”)
Specific person: Neked tilos enni a könyvtárban. or with a personal infinitive: Tilos enned a könyvtárban. Personal infinitive endings with enni: ennem (I), enned (you sg), ennie (he/she), ennünk, ennetek, enniük.

Why is the infinitive enni and not something like eszni?

“Eat” is irregular: the dictionary form/infinitive is enni, but the present-tense stem is eszik:

I eat: eszem; you: eszel; he/she: eszik
Past: ettem, ettél, evett, etc. A similar pattern exists with inni (“to drink”): present iszik.

How do I pronounce this sentence?

Stress is always on the first syllable of each word: KÖNYV-tár-ban, TI-los, EN-ni.
ö is a front rounded vowel (like French “peu”).
ny is a single sound, like the “ny” in “canyon” (palatal n).
á is a long “a” (as in “father,” length matters). A rough guide: “KÖNYV-tar-ban TEE-losh EN-ni.”

How do I say “In libraries it’s forbidden to eat” (plural)?

A könyvtárakban tilos enni. (könyvtárak = libraries; -ban attached to the plural form: könyvtárakban)

Can I use a noun instead of the infinitive, like “Eating is forbidden in the library”?

Yes: Az evés tilos a könyvtárban. It’s correct and slightly more formal. On signs and in everyday use, the infinitive Enni tilos is extremely common.

How do I ask for permission politely?

Szabad enni a könyvtárban? (“Is it allowed to eat in the library?”)
Lehet enni a könyvtárban? (“Is it possible/allowed to eat in the library?”)
Polite notice style: Kérjük, ne egyenek a könyvtárban. (“Please do not eat in the library.”; plural/formal “you”)

What does Nem tilos enni a könyvtárban mean, and is it the same as “allowed”?

Nem tilos enni a könyvtárban = “It’s not forbidden to eat in the library.” That only states there is no prohibition. If you want to say it is allowed, use Szabad enni a könyvtárban or colloquially Lehet enni a könyvtárban.

Is there a difference between tilos and tiltott?

Yes.

tilos = “forbidden” used predicatively about actions: Tilos enni.
tiltott = “forbidden/prohibited” as an attributive adjective before nouns: tiltott tárgy (“prohibited item”). You don’t normally say tiltott enni.

Why do signs sometimes just say Enni tilos! without the place?

That’s a concise, sign-style imperative-like statement meaning “No eating!” The place is understood from context (e.g., posted inside a library). You can also see: Dohányozni tilos! (“No smoking!”).