Prima di usare il coltello, lava le mani.

Because prima di means before when it is followed by a verb in the infinitive.

• prima di + infinitive = before doing something
• prima di usare = before using

This is a very common pattern in Italian:

• Prima di uscire, chiudi la porta. = Before going out, close the door.
• Prima di mangiare, lavati le mani. = Before eating, wash your hands.

If you want to say before followed by a noun, you can also use prima di:

• prima di cena = before dinner

Because after the preposition di, Italian normally uses the infinitive when the subject is the same or understood.

So:

• prima di usare = before using
• not a fully conjugated clause like English sometimes allows

English can say:

• Before you use the knife, wash your hands.
• Before using the knife, wash your hands.

Italian prefers the infinitive structure here:

• Prima di usare il coltello, lava le mani.

If you wanted a full clause with an explicit subject, Italian would usually use a different structure, for example:

• Prima che tu usi il coltello... = Before you use the knife...

But that is less direct and not the best match for this sentence.

Lava here is the informal singular imperative of lavare.

It is the form you use when giving a command to tu:

• lava! = wash!
• mangia! = eat!
• ascolta! = listen!

So lava le mani means wash your hands when speaking to one person informally.

A learner may notice that lava also looks like the present tense form meaning he/she washes. That is true, but in this sentence the context clearly makes it a command.

It is talking to one person informally.

That is because lava is the command form for tu.

Other versions would be:

• Lava le mani. = informal, one person
• Lavate le mani. = informal, more than one person
• Lavi le mani. = formal, one person

So if you were speaking politely to someone you do not know well, you would say:

• Prima di usare il coltello, lavi le mani.

And to a group:

• Prima di usare il coltello, lavate le mani.

Italian usually uses an article more often than English does.

So il coltello simply means the knife. In context, it can refer to:

• a specific knife
• the knife you are about to use
• the knife as the relevant object in the situation

Italian generally does not drop the article here. A bare noun like usare coltello would be ungrammatical.

Italian often uses the definite article with body parts where English uses a possessive.

So:

• lava le mani literally = wash the hands
• natural English = wash your hands

This is very common in Italian, especially with body parts:

• Mi lavo le mani. = I wash my hands.
• Alza la testa. = Raise your head.
• Chiudi gli occhi. = Close your eyes.

Using le tue mani is possible, but it is usually more emphatic or contrastive, not the default everyday wording.

A native English speaker is very likely to wonder about this, and it is a good question.

In everyday Italian, when you mean wash your hands, the most natural form is often:

• Lavati le mani.

That is the imperative of lavarsi (to wash oneself).

So many Italians would naturally say:

• Prima di usare il coltello, lavati le mani.

The version in your sentence, lava le mani, is understandable, but it sounds less idiomatic if the meaning is specifically wash your own hands. It sounds more like a plain command with the object the hands, where the person affected is understood from context.

So for learners, the most useful takeaway is:

• lavati le mani = the most natural way to say wash your hands
• lava le mani = understandable, but less typical in this exact meaning

It comes from the reflexive verb lavarsi.

• infinitive: lavarsi = to wash oneself
• imperative for tu: lava
• attach the reflexive pronoun ti to the end: lavati

Then add the object:

• Lavati le mani.

This literally works like:

• wash-yourself the hands

But in normal English, of course, we say:

• wash your hands

This structure is very common with body-care actions:

• Mettiti il cappotto. = Put on your coat.
• Pettinati i capelli. = Brush your hair.
• Asciugati le mani. = Dry your hands.

The comma separates the introductory phrase from the main command.

• Prima di usare il coltello = introductory time phrase
• lava le mani = main clause

English works similarly:

• Before using the knife, wash your hands.

The comma is natural and helpful here, especially because the sentence begins with a longer introductory phrase.

You could also put the time phrase at the end:

• Lava le mani prima di usare il coltello.

That version usually does not need a comma.

Yes. Italian allows both of these:

• Prima di usare il coltello, lava le mani.
• Lava le mani prima di usare il coltello.

Both mean the same thing.

The first version emphasizes the before using the knife part first. The second version starts with the command itself.

Both are natural, though the second may feel a bit more straightforward in everyday speech.

If you want to address someone formally, Italian uses the Lei command form.

So the formal version is:

• Prima di usare il coltello, lavi le mani.

And if you want the more idiomatic reflexive version:

• Prima di usare il coltello, si lavi le mani.

That second one is especially useful to know, because it matches the very common pattern of using a reflexive verb with body parts.

Usually yes, in this kind of sentence.

• prima di + noun = before + noun
• prima di + infinitive = before + -ing / before doing

Examples:

• prima di pranzo = before lunch
• prima di partire = before leaving

Just remember that Italian uses di here, not a direct equivalent of English before + subject + verb unless you change the structure.

So prima di usare is one of the most useful patterns to memorize as a chunk.