Děti si musí čistit zuby po večeři.

Děti is the plural of dítě, which means child.

This is an irregular noun, so the plural is not something simple like adding an ending to the singular in an obvious way.

• singular: dítě = child
• plural: děti = children

In the sentence, Děti is the subject, so it is in the nominative plural.

Because the verb muset has the same form in the 3rd person singular and 3rd person plural in the present tense.

So:

• on musí = he must / has to
• ona musí = she must / has to
• ono musí = it must / has to
• oni musí = they must / have to
• ony musí = they must / have to
• děti musí = children must / have to

This often surprises English speakers, because English also uses the same form in he must and they must, but learners may expect Czech plural agreement to look different. Here, it just happens that the forms are identical.

Si is the reflexive pronoun in the dative.

In čistit si zuby, it shows that the children are brushing their own teeth. Czech very often uses this reflexive structure with body parts and personal care actions.

So:

• čistit si zuby = to brush one’s teeth
  literally something like to clean teeth to oneself

This is a very common Czech pattern.

Because with body parts, Czech usually does not use a possessive adjective when it is obvious whose body part it is. Instead, it uses the reflexive pronoun si.

So Czech prefers:

• Děti si čistí zuby. = The children are brushing their teeth.

rather than something like:

• Děti čistí své zuby.

The version with své is grammatically possible, but it is less natural in an ordinary sentence like this. The reflexive si is the normal choice.

Because after the modal verb muset (must / have to), Czech uses an infinitive.

So the structure is:

• musí + infinitive

Here:

• musí čistit = must brush / have to brush

This is similar to English:

• must brush
• have to brush

You do not conjugate the second verb here; only musí is conjugated.

Čistit is the imperfective verb, and vyčistit is the perfective partner.

In this sentence, čistit is used because the meaning is general or habitual: it describes a regular obligation.

• Děti si musí čistit zuby po večeři.
  = Children have to brush their teeth after dinner.
  (general rule / repeated action)

If you said:

• Děti si musí vyčistit zuby.

that would sound more like The children must get their teeth brushed / must finish brushing their teeth, with a stronger sense of completion in a specific instance.

So for routine habits, čistit si zuby is very natural.

Zuby is the accusative plural of zub (tooth).

The verb čistit takes a direct object, so zuby has to be in the accusative.

• singular: zub
• plural nominative: zuby
• plural accusative: zuby

For this noun, the nominative plural and accusative plural look the same, so the form does not change visibly.

Here, po means after, and in this meaning it takes the locative case.

So:

• večeře = dinner
• po večeři = after dinner

This is a very useful pattern to remember:

• po obědě = after lunch
• po práci = after work
• po škole = after school

So in this sentence, večeři is locative singular.

It can correspond to both.

• muset = must / have to

In many everyday contexts, English speakers translate it more naturally as have to, because must can sound stronger or more formal in English.

So:

• Děti si musí čistit zuby po večeři. can mean
• Children must brush their teeth after dinner.
• Children have to brush their teeth after dinner.

Both are fine; the best choice depends on tone.

Because Czech does not have articles like a or the.

So:

• děti can mean children or the children
• večeře can mean dinner or the dinner

Context tells you whether the meaning is general or specific.

In this sentence, Děti could mean:

• Children in general, or
• the children in a specific family/group

Both are possible depending on context.

Yes. Czech word order is more flexible than English word order.

The sentence:

• Děti si musí čistit zuby po večeři.

is a neutral, natural order.

But you could also say:

• Po večeři si děti musí čistit zuby.

This puts after dinner first, which may emphasize the time condition more.

The meaning stays basically the same; the difference is mostly about emphasis and information structure.

Yes, very much so.

Čistit si zuby is the normal Czech way to say brush one’s teeth.

Even though čistit literally means to clean, this is the standard expression for tooth-brushing.

So learners should treat it as a common phrase:

• čistit si zuby = brush one’s teeth

That is more useful than translating each word too literally.