Prima di bere acqua fresca, devo risciacquare bene il bicchiere.

Because Italian normally uses prima di + infinitive to mean before doing something.

• prima di bere = before drinking
• prima di uscire = before going out
• prima di parlare = before speaking

So di is required here before the infinitive bere.

After prima di, Italian uses the infinitive, not a conjugated verb.

So:

• prima di bere = before drinking
• not prima di bevo

This is similar to English structures like before eating or before leaving, where you are talking about the action in a general way rather than saying who is doing it in a separate clause.

Prima di is followed by an infinitive when the subject is the same or when Italian simply uses an infinitive construction.

• Prima di bere acqua fresca, devo risciacquare bene il bicchiere.

Prima che is followed by a full clause with a conjugated verb, usually when a different subject is involved.

• Prima che tu beva acqua fresca, devo risciacquare il bicchiere.

Also, after prima che, Italian normally uses the subjunctive:

• prima che io vada
• prima che lui arrivi

So in your sentence, prima di bere is the natural and simpler structure.

In Italian, articles are often omitted when talking about something in a general, uncountable sense.

Here, bere acqua fresca means to drink fresh/cool water in a general sense, not the fresh water or some specific water.

Compare:

• bere acqua fresca = to drink fresh water / cool water
• bere l’acqua fresca = to drink the fresh water, a specific water already known in context

So the lack of article is completely normal.

Fresca is the feminine singular form of fresco, and here it means fresh or more naturally cool.

Because acqua is feminine singular, the adjective must agree:

• acqua fresca
• vino fresco
• bibite fresche

In many everyday contexts, acqua fresca often means cool water, not necessarily freshly obtained water.

Italian usually drops subject pronouns when they are not needed, because the verb ending already shows the subject.

• devo already means I must
• devi = you must
• deve = he/she/it must

So io is optional. You could say io devo for emphasis, contrast, or clarity, but in a neutral sentence devo is enough.

Risciacquare means to rinse.
Lavare means to wash.

So:

• lavare il bicchiere = to wash the glass
• risciacquare il bicchiere = to rinse the glass

In your sentence, the idea is specifically rinsing the glass well before drinking.

Bene is an adverb meaning well, and in Italian adverbs like this often come after the verb.

• risciacquare bene il bicchiere = to rinse the glass well

This placement is very natural. Italian word order with adverbs is often more flexible than English, but this is one of the most common patterns.

Il bicchiere means the glass, while un bicchiere means a glass.

Italian often uses the definite article where English might also use the, especially when referring to a concrete object involved in the situation. Here it sounds like a specific glass that the speaker is going to use.

• devo risciacquare il bicchiere = I have to rinse the glass
• devo risciacquare un bicchiere = I have to rinse a glass, any glass / one glass

So il bicchiere suggests a particular glass in context.

The comma separates the introductory phrase from the main clause.

• Prima di bere acqua fresca = introductory time phrase
• devo risciacquare bene il bicchiere = main clause

The comma helps readability and is very natural in writing, especially when the opening phrase is a bit longer. In casual writing, some people might omit it, but with this structure the comma is perfectly standard.

Yes. That version is also correct and natural.

• Prima di bere acqua fresca, devo risciacquare bene il bicchiere.
• Devo risciacquare bene il bicchiere prima di bere acqua fresca.

The meaning is basically the same. The first version emphasizes the before drinking part first, while the second starts with the main action I must rinse the glass well.

Yes, it is grammatically correct and natural. However, depending on context, native speakers might interpret acqua fresca as cool water more than fresh water in the English sense.

If you specifically mean fresh water as opposed to salt water, Italian often says:

• acqua dolce = fresh water

But in an everyday sentence about drinking, acqua fresca is very normal and usually means pleasantly cool drinking water.

Italian adjectives usually agree in gender and number with the noun they describe.

Since acqua is feminine singular, the adjective must also be feminine singular:

• acqua fresca
• bevanda fresca

If the noun were masculine singular, you would use fresco:

• vino fresco

If plural, the adjective changes too:

• acque fresche
• bicchieri puliti

This kind of agreement is one of the key differences between Italian and English.

Devo can cover both I must and I have to, depending on context and tone.

• devo risciacquare il bicchiere can mean I must rinse the glass or I have to rinse the glass

Italian dovere is very common for obligation, necessity, or something the speaker feels is required. The exact strength often depends on the situation rather than the verb alone.