No parlis tan fort a l'hospital.

Because Catalan uses the present subjunctive for negative commands.

• Positive command to one person you know well: Parla! = Speak!
• Negative command: No parlis! = Don’t speak!

So in your sentence:

• No parlis tan fort a l'hospital.

This is the normal way to say Don’t speak so loudly in the hospital when talking to tu.

Parlis is the second person singular informal form, used with tu.

So the hidden subject is:

• (tu) no parlis

If you were speaking to:

• vostè: No parli tan fort
• vosaltres: No parleu tan fort
• vostès: No parlin tan fort

Tan means so in the sense of to this degree.

So:

• tan fort = so loudly / so loud

This is different from molt fort, which usually means very loud.

Compare:

• No parlis tan fort. = Don’t speak so loudly.
• Parla molt fort. = He/She speaks very loudly.

A native English speaker may confuse tan and molt, but here tan is the natural choice for so loud.

Yes, fort often means strong, but it can also mean loud depending on context.

With verbs like parlar (to speak), fort commonly means:

• parlar fort = to speak loudly

So in this sentence, fort is not about physical strength. It describes the volume of the speech.

Because here fort is working like an adverbial expression, describing how someone speaks.

• parlar fort = to speak loudly
• cridar fort = to shout loudly

Even though fort is originally an adjective, in expressions like this it stays in the same form and does not agree with anything.

A l'hospital is made of:

• a = at / to / in depending on context
• l' = the elided form of the article el
• hospital = hospital

So:

• a + el hospital becomes a l'hospital

This happens because hospital begins with a vowel sound.

Compare:

• al parc = at/to the park
• a l'escola = at/to the school
• a l'hospital = at/in/to the hospital

Catalan often uses the definite article where English may or may not use the.

So a l'hospital literally looks like at the hospital, and that is also the normal Catalan way to express this idea.

English sometimes says:

• in hospital (especially British English)
• in the hospital (especially American English)

Catalan normally uses the article here:

• a l'hospital

It can mean different things depending on context, because a is flexible in Catalan.

In this sentence, the natural meaning is at/in the hospital, because it describes where the speaking is happening:

• No parlis tan fort a l'hospital. = Don’t speak so loudly at/in the hospital.

If the sentence were about movement, then a could mean to.

It goes directly before the verb:

• No parlis
• No corris
• No mengis això

This is similar to English don’t + verb, but Catalan does not use an extra helping verb like do.

So instead of something like don’t speak, Catalan simply says:

• no parlis

A simple approximate pronunciation for an English speaker is:

• noh PAR-lees tan FORT ah luz-pee-TAHL

A few helpful notes:

• parlis: the i is a clear vowel, not like the English eye
• tan: short and crisp
• fort: the r is Catalan/Spanish-style, not an English r
• l'hospital: the h is silent, so it sounds like lospital

A more Catalan-like pronunciation would be roughly:

• no PAR-lis tan FORT a lus-pi-TAL

Yes, that is grammatical, but it means something slightly different.

• No parlis tan fort = Don’t speak so loudly
• No parlis molt fort = Don’t speak very loudly

In many situations, tan fort sounds more natural if you are reacting to someone who is being louder than is appropriate.

So if someone is disturbing people, tan fort is often the best choice.

The positive tu command is:

• Parla més fluix a l'hospital. = Speak more quietly in the hospital.
• or simply Parla fluix. = Speak quietly.

If you keep the same verb but make it a direct positive command, it would be:

• Parla.

So the contrast is:

• Parla! = Speak!
• No parlis! = Don’t speak!

This contrast is a very important pattern in Catalan.