אסור לאכול באוטובוס, אבל מותר לשתות מים מבקבוק.

They mean forbidden and allowed / permitted.

In this pattern:

• אסור + infinitive = it is forbidden to... / you must not...
• מותר + infinitive = it is allowed to... / you may...

So:

• אסור לאכול = It is forbidden to eat
• מותר לשתות = It is allowed to drink

These words are often used impersonally in Hebrew, especially in rules, signs, and instructions.

Because they are infinitives, and Hebrew infinitives usually begin with ל־, which often corresponds to English to.

So:

• לאכול = to eat
• לשתות = to drink

In this sentence, אסור and מותר are followed by an infinitive:

• אסור לאכול = forbidden to eat
• מותר לשתות = allowed to drink

Because it is made from:

• ב־ = in / at / on
• ה־ = the
• אוטובוס = bus

When ב־ combines with ה־, they usually merge into בַּ / בָּ / בֶּ in normal spelling without vowels, so you get:

• ב + האוטובוס → באוטובוס

So באוטובוס means in the bus / on the bus.

Hebrew usually uses ב־ with vehicles like buses, cars, trains, and planes where English often says on.

So English says:

• on the bus
• on the train

But Hebrew commonly says:

• באוטובוס
• ברכבת

Literally, ב־ often means in, but in these cases the natural English translation is usually on.

Because את is used before a definite direct object, and מים here is not definite.

Compare:

• לשתות מים = to drink water
  general, non-specific water → no את
• לשתות את המים = to drink the water
  specific water → את is needed

So in your sentence, מים means just water, not the water.

It means from a bottle.

It is made from:

• מ־ = from
• בקבוק = bottle

So:

• מבקבוק = from a bottle

In the full sentence, לשתות מים מבקבוק means to drink water from a bottle.

Without ה־, it usually means from a bottle in a general sense.

• מבקבוק = from a bottle
• מהבקבוק = from the bottle

So this sentence is saying that drinking water from a bottle is allowed in general, not necessarily from one specific bottle.

מים looks plural, and historically it is a plural form, but in modern Hebrew it functions as a mass noun meaning water.

A learner should mostly just remember:

• מים = water

You will often see it treated grammatically as plural in some contexts, but in everyday learning, it is best to memorize it as the normal Hebrew word for water.

Not in this kind of sentence.

Here they are being used impersonally, as in a rule or general statement:

• אסור לאכול = It is forbidden to eat
• מותר לשתות = It is allowed to drink

This does not change depending on whether the person is male, female, singular, or plural.

However, in other sentence types, מותר and אסור can agree with nouns. But in this common rule + infinitive structure, learners can treat them as fixed forms.

אבל means but.

It connects the two contrasting parts:

• אסור לאכול באוטובוס = Eating on the bus is forbidden
• אבל = but
• מותר לשתות מים מבקבוק = drinking water from a bottle is allowed

Its placement is very similar to English: it comes between the two clauses.

A simple pronunciation guide is:

ah-SOOR le-eh-KHOL ba-o-to-BOOS, a-VAL moo-TAR lish-TOT MA-yim mi-bak-BOOK

A more readable transliteration:

Asur le’ekhol ba’otobus, aval mutar lishtot mayim mibakbuk.

Approximate stress:

• אָסוּר → ah-SOOR
• לֶאֱכוֹל → le-eh-KHOL
• בָּאוֹטוֹבּוּס → ba-o-to-BOOS
• אֲבָל → a-VAL
• מוּתָר → moo-TAR
• לִשְׁתּוֹת → lish-TOT
• מַיִם → MA-yim
• מִבַּקְבּוּק → mi-bak-BOOK

Yes, it is natural and clear.

It sounds like a rule or notice, such as something you might see on public transportation. Hebrew often expresses rules this way:

• אסור...
• מותר...

So this sentence is a very normal pattern for learners to know.

A slightly more formal or sign-like version might also use other structures, but your sentence is completely natural.