Isperite usta vodom.

Isperite is the imperative form, meaning it is used to give a command or instruction.

More specifically, it is:

• 2nd person plural: speaking to more than one person
• or formal/polite singular: speaking politely to one person

So this is the form you would expect in instructions, medical directions, labels, or polite speech.

Related forms:

• isperi = singular informal command
• isperite = plural or formal command

This verb changes its stem in different forms.

The dictionary form is isprati, but the present/imperative stem is isper-:

• isprati = infinitive
• isperem = I rinse
• isperi = rinse! (informal singular)
• isperite = rinse! (plural/formal)

This is the same pattern as:

• prati → perem, peri
• oprati → operem, operi

So isprati → isperite is a normal stem change, not an irregular typo or exception specific to this sentence.

Because usta in Serbian is a plural-only noun for mouth. This is one of those nouns that look plural even when English uses a singular word.

So:

• usta = mouth

It does not mean mouths in this sentence.

This is something learners just have to get used to. Serbian sometimes uses a plural form where English uses a singular idea.

Here usta is in the accusative case, because it is the direct object of the verb isperite.

You are rinsing what?

• usta

For this noun, the nominative and accusative forms are the same:

• usta

So even though the form does not change, its role in the sentence is still accusative.

Because vodom is the instrumental singular of voda.

The instrumental case is often used to show:

• the means
• the substance used
• the tool used to do something

So vodom means roughly:

• with water
• using water

That is why the sentence uses:

• voda = water
• vodom = with water

In Serbian, possessive words like your are often omitted when the meaning is obvious from context.

So Isperite usta vodom naturally means:

• rinse your mouth with water

Serbian does not need to say your here, because it is understood from the command itself.

If you added a possessive, it would usually sound unnecessary unless you were emphasizing or contrasting something.

It can be either.

The ending -te is used for:

• speaking to more than one person
• speaking politely to one person

So without more context, isperite could mean:

• you all rinse
• please rinse (to one person, politely)

This is very common in Serbian imperatives.

This is a question of aspect.

• isperite is perfective
• ispirajte is imperfective

Very roughly:

• isperite = rinse it, do the action as a complete whole
• ispirajte = keep rinsing / rinse repeatedly / be in the process of rinsing

In instructions like this, perfective is very common because it presents the action as one complete step.

So Isperite usta vodom sounds like a standard instruction: do this action and complete it.

In standard Serbian, vodom is the natural choice here.

That is because the sentence is expressing the means by which the action is done, and Serbian usually uses the instrumental without a preposition for that:

• Isperite usta vodom.

Using sa vodom here is generally less natural in standard phrasing. A learner should prefer vodom in this type of sentence.

Yes. Serbian word order is fairly flexible.

But Isperite usta vodom is the most neutral and natural order for an instruction:

• verb first
• then object
• then the means/instrument

You might hear other orders for emphasis, but this version is the best basic model to learn first.