Penso che la tassa di soggiorno sia già scritta sul conto.

Because penso che usually introduces a clause that takes the subjunctive in standard Italian.

• Penso che ... sia = I think that ... is
• After verbs of opinion, doubt, feeling, and possibility, Italian often uses the subjunctive:
  • Penso che sia vero.
  • Credo che abbiano ragione.

So in this sentence, sia is the expected standard form.

Sia is the present subjunctive of essere.

Here is the present subjunctive of essere:

• che io sia
• che tu sia
• che lui/lei sia
• che noi siamo
• che voi siate
• che loro siano

In the sentence la tassa di soggiorno sia già scritta sul conto, sia means is.

In standard Italian, yes, it normally does.

• Penso che sia giusto. = standard
• Penso che è giusto. = sometimes heard in casual speech, but generally not recommended for learners

So if you are learning correct, broadly accepted Italian, use subjunctive after penso che.

Tassa di soggiorno is the tax some cities or towns charge visitors staying in hotels, B&Bs, or other accommodation.

In English, it is often translated as:

• tourist tax
• city tax
• lodging tax or occupancy tax in some contexts

Literally:

• tassa = tax
• soggiorno = stay

So it is the tax connected with your stay.

Italian usually uses the definite article much more often than English.

So:

• la tassa di soggiorno = the tourist tax / the city tax

Even when English would use no article, Italian often still uses one with nouns like this. That is normal and idiomatic.

Because scritta agrees with la tassa, which is feminine singular.

• tassa = feminine singular
• therefore the past participle/adjective must also be feminine singular:
  • scritta

Compare:

• il prezzo è scritto → masculine singular
• la tassa è scritta → feminine singular
• le tasse sono scritte → feminine plural

Yes, scritta literally means written. In this sentence it means something like written on the bill, listed on the bill, or shown on the bill.

A native speaker might also say:

• indicata sul conto = indicated/listed on the bill
• segnata sul conto = marked on the bill
• inclusa nel conto = included in the bill

So scritta is understandable, but depending on the exact nuance, another word might sound a bit more natural.

Here conto means bill or check, especially in a hotel or restaurant context.

So:

• sul conto = on the bill

Be careful: conto can also mean account in other contexts, but here it clearly means the amount/bill presented for payment.

Both prepositions are possible in Italian, but they suggest slightly different ideas.

• sul conto = on the bill, meaning it appears there as a written item
• nel conto = in the bill, meaning it is included within the total

So:

• scritta sul conto focuses on it being written/listed there
• inclusa nel conto focuses on it being included in the amount charged

That is why sul conto works well with scritta.

Già means already.

So:

• sia già scritta = is already written / already listed

It suggests that the tax has already been added or appears there already, so perhaps there is no need to add it separately.

In Italian, adverbs like già often go before the adjective or participle they modify.

So:

• sia già scritta = is already written

That word order is very natural. Italian word order is flexible, but this placement is the most straightforward here.

You might hear that in informal speech, but it is not the standard choice.

For learners, the correct form is:

• Penso che la tassa di soggiorno sia già scritta sul conto.

Use è only if the structure changes and no subjunctive trigger is present, for example:

• La tassa di soggiorno è già scritta sul conto. = The tourist tax is already written on the bill.

Yes. Penso che and credo che are both common.

• Penso che la tassa di soggiorno sia già scritta sul conto.
• Credo che la tassa di soggiorno sia già scritta sul conto.

They are very similar, though:

• penso can feel a bit more like I think
• credo can feel a bit more like I believe / I think

In both cases, the following clause normally takes the subjunctive.

Not exactly.

• conto = bill/check, often in restaurants, hotels, everyday payment situations
• fattura = invoice, a more formal billing document
• ricevuta = receipt

In this sentence, conto makes sense if someone is talking about what appears on the bill they have to pay.

Yes. Sia scritta is basically a passive-style expression here.

It means the tax is written / is listed on the bill.

Italian often uses past participles like this with essere to describe how something appears:

• Il nome è scritto male. = The name is written badly.
• La tassa è scritta sul conto. = The tax is written on the bill.

So you can think of it as is written or more naturally in English is listed.