Prvo koristite konac za zube, onda isperite usta.

Koristite is the vi form. In Serbian, that can mean either:

• you all (plural), or
• you to one person in a polite/formal way

So this sentence can be addressing several people, or one person politely.

If you were speaking to one friend or family member informally, you would usually say:

• Prvo koristi konac za zube, onda isperi usta.

Here it is understood as a command/instruction.

The form koristite can look the same as the present tense you use, but context makes the meaning clear. In a step-by-step instruction like this, Serbian often uses forms like koristite and isperite as directions:

• Prvo koristite ... = first use ...
• onda isperite ... = then rinse ...

So even though the form may match the present tense, here it functions as an instruction.

Yes. Both are natural and mean dental floss.

• konac za zube = literally thread for teeth
• zubni konac = dental floss, more compact

A native speaker would understand either one. Konac za zube is very transparent for learners because you can see the structure clearly.

After za meaning for, Serbian uses the accusative here.

• dictionary form: zub = tooth
• accusative plural: zube

So:

• konac za zube = floss for teeth

This pattern is very common:

• četkica za zube = toothbrush
• pasta za zube = toothpaste

Because usta is a plural-only noun in Serbian when it means mouth.

So even when talking about one person’s mouth, Serbian normally says usta, not a singular form. That is why:

• isperite usta = rinse your mouth

Grammatically it behaves like a plural noun, even though the meaning in English is often singular.

In Serbian, possessives are often left out when they are obvious, especially with body parts.

So:

• isperite usta naturally means rinse your mouth
• not someone else’s mouth

If you added a possessive, it would usually be for emphasis or contrast:

• isperite svoja usta = rinse your own mouth

In normal instructions, that extra word is unnecessary.

They show the order of actions:

• prvo = first
• onda = then

So the sentence is structured very clearly as a sequence.

You could also hear alternatives such as:

• najpre / najprije = first
• zatim = then / next
• pa = then / and then, more conversational

For example:

• Najpre koristite konac za zube, zatim isperite usta.
• Prvo koristite konac za zube, pa isperite usta.

All of these are possible.

This is a question of aspect, which is very important in Serbian verbs.

• isperite is perfective
• ispirajte is imperfective

Here, isperite fits well because the sentence gives a single completed step in a sequence:

• first do one thing
• then complete the rinsing action

Using ispirajte would sound more like:

• rinse repeatedly
• keep rinsing
• rinse as an ongoing activity

In step-by-step instructions, perfective imperatives like isperite are very common.

Because some Serbian verbs change their stem in the present tense and imperative.

So:

• infinitive: isprati
• present: isperem, ispereš, ispere...
• imperative: isperi, isperite

This kind of change is normal in Serbian and has to be learned with the verb. Another similar example is:

• oprati → operi

So isperite may look irregular at first, but it follows a common pattern.

You would usually say:

• Prvo koristi konac za zube, onda isperi usta.

So the difference is:

• koristite / isperite = plural or polite/formal
• koristi / isperi = singular informal

That is one of the most useful things to notice in this sentence.