Hele, publikum začíná tleskat do rytmu.

Hele is a very common informal Czech attention-getter. In this sentence it works like:

• Hey
• Look
• Listen

It does not literally describe seeing something every time; often it just draws the listener’s attention to what comes next.

A few notes:

• It is colloquial, not very formal.
• You hear it a lot in spoken Czech.
• In more formal speech, you would usually leave it out or use a different structure.

So Hele, publikum začíná tleskat do rytmu. is something like Hey, the audience is starting to clap in rhythm.

Publikum means the audience as a collective noun, just like English audience can refer to many people as one group.

In Czech, publikum is:

• a singular noun
• neuter
• treated grammatically as one unit

That is why the verb is singular:

• publikum začíná = the audience is starting

Even though the audience contains many people, Czech grammar treats publikum here as one collective whole.

It is in the nominative, because it is the subject of the sentence.

You can see that in the structure:

• publikum = subject
• začíná = verb
• tleskat = infinitive dependent on začíná

So the basic pattern is:

• [subject] + začíná + infinitive
• publikum začíná tleskat

Because začínat means to begin / to start, and in Czech it is commonly followed by an infinitive when you say someone starts doing something.

Pattern:

• začít / začínat + infinitive

Examples:

• Začínám pracovat. = I’m starting to work.
• Děti začaly zpívat. = The children started to sing.
• Publikum začíná tleskat. = The audience is starting to clap.

This is very similar to English start doing or start to do.

This is about aspect.

• začínat = imperfective = to be starting / to start in progress or generally
• začít = perfective = to start / to begin as a completed event

So:

• publikum začíná tleskat suggests the action is just getting underway, unfolding now
• publikum začne tleskat would sound more like the audience will start clapping or starts clapping as a single event

In your sentence, začíná fits the idea of something happening right now before your eyes.

Tleskat means to clap or to applaud, depending on context.

Here it means physically clapping one’s hands.

Useful related forms:

• tleskat = to clap
• zatleskat = to clap once / to give applause as a completed action
• potlesk = applause

A useful grammar point: tleskat often takes the dative when you clap for someone:

• tleskat herci = to applaud the actor

But in your sentence, there is no person being applauded; instead, we have the phrase do rytmu, which tells us how they are clapping.

Do rytmu means to the rhythm / in rhythm / along with the beat.

Literally:

• rytmus / rytmus? The standard noun is rytmus
• after do, it becomes do rytmu because do requires the genitive

So:

• rytmus = rhythm
• do rytmu = into/to the rhythm

In natural English, you would usually translate it as:

• in rhythm
• to the beat
• in time with the rhythm

This is a fixed and common Czech expression:

• tleskat do rytmu = to clap in rhythm / clap along to the beat

Because the preposition do requires the genitive case.

Dictionary form:

• rytmus = rhythm

Genitive singular:

• rytmu

So:

• do + genitive
• do rytmu

This kind of change is very common in Czech. The noun ending changes because its grammatical role changes.

The given word order is natural and neutral for spoken Czech:

• Hele, publikum začíná tleskat do rytmu.

But Czech word order is flexible, so other versions are possible depending on emphasis.

For example:

• Hele, začíná publikum tleskat do rytmu.
  This is possible, but less neutral and may sound more stylistically marked.
• Publikum začíná tleskat do rytmu.
  Same sentence without the informal attention-getter.

In ordinary conversation, the original order is the most natural.

A rough pronunciation guide:

• Hele ≈ HEH-leh
• publikum ≈ POOB-lih-koom
• začíná ≈ za-CHEE-naa
• tleskat ≈ TLES-kat
• do rytmu ≈ do RIT-moo

A few points:

• ř does not appear here, so there is nothing especially difficult by Czech standards.
• tleskat begins with the consonant cluster tl-, which may feel unusual for English speakers.
• Stress in Czech is usually on the first syllable of each word:
  • HE-le
  • PU-blikum
  • ZA-číná
  • TLES-kat
  • DO rytmu

It is mostly neutral spoken Czech, but with a clearly informal opening because of Hele.

• Hele = casual, conversational
• publikum začíná tleskat do rytmu = otherwise normal and standard Czech

So the whole sentence sounds like something you would say in everyday speech, for example when watching a concert or performance with someone.

Without Hele, it becomes more neutral:

• Publikum začíná tleskat do rytmu.

Yes. Depending on context, Czech could use a few alternatives, for example:

• v rytmu = in rhythm
• do taktu = to the beat
• v taktu = in time

But tleskat do rytmu is very idiomatic and natural when people are clapping along with music or a beat.

So in this sentence, do rytmu is a very good and natural choice.