Když zazvoní zvonek u dveří, můj pes hned běží ke dveřím.

Když means when. It introduces a time clause:

• Když zazvoní zvonek u dveří = When the doorbell rings
• můj pes hned běží ke dveřím = my dog immediately runs to the door

So the whole sentence has two parts:

1. the event that happens first
2. the result or reaction

Because the když-clause comes first, Czech also uses a comma before the main clause.

In Czech, a subordinate clause introduced by když is separated by a comma from the main clause.

So:

• Když zazvoní zvonek u dveří, můj pes hned běží ke dveřím.

This is the normal punctuation rule, just like English often uses a comma after an introductory when-clause:

• When the doorbell rings, my dog immediately runs to the door.

This is mainly about aspect.

• zvonit = to ring, to be ringing, to ring repeatedly or as a process
• zazvonit = to ring once / to start ringing / to give a ring

So:

• zvonek zvoní = the bell is ringing
• zvonek zazvoní = the bell rings / goes off

In this sentence, the idea is a single completed trigger event: the bell goes off, and then the dog reacts. That is why zazvoní is very natural here.

Formally, it looks like a present-tense form, but because zazvonit is a perfective verb, its present form usually has future meaning.

So zazvoní can mean:

• will ring
• rings in the sense of whenever it rings / when it rings

In this sentence, English uses simple present for a general repeated situation:

• When the doorbell rings, my dog immediately runs to the door.

Czech commonly uses a perfective verb in this kind of clause to mark the event as a completed trigger.

Běží is from běžet = to run.

Here it is in the present tense because the sentence describes a general habitual reaction:

• whenever the bell rings, the dog runs to the door

So Czech, like English, can use the present tense for something that happens regularly.

Hned means immediately, right away, or at once.

So:

• můj pes hned běží ke dveřím
  = my dog immediately runs to the door

It shows that the dog reacts very quickly after hearing the bell.

Zvonek is the subject of the clause zazvoní zvonek u dveří, so it is in the nominative singular.

Base form:

• zvonek = bell, doorbell

Because it is the thing doing the action (ringing), nominative is used.

Literally, it means the bell by the door or the doorbell at the door.

• zvonek = bell / doorbell
• u = by, near, at
• dveří = of the door / doors, after u

In natural English, you would usually just say the doorbell.

Czech often expresses this as zvonek u dveří, meaning the bell located by the door.

This is because different prepositions require different cases.

• u takes the genitive
• k / ke takes the dative

So:

• u dveří = by the door / at the door
• ke dveřím = to the door

The two phrases have different meanings:

• u dveří describes location
• ke dveřím describes movement toward something

Because u requires the genitive case.

The noun is dveře = door, but in Czech this word is normally used only in the plural form.

Its genitive plural is:

• dveří

So:

• u dveří = by the door

Because k / ke requires the dative case.

The dative plural of dveře is:

• dveřím

So:

• ke dveřím = to the door

This is a very common pattern in Czech:

• jdu ke škole = I am going to the school
• běží ke dveřím = he/she runs to the door

Dveře is one of those Czech nouns that are normally plural in form, even though in English the meaning is often singular.

So:

• dveře = door
• but grammatically it behaves like a plural noun

That is why you get forms like:

• u dveří
• ke dveřím

This is something learners just have to get used to. It is similar to how some English nouns do not behave exactly as expected, though the Czech pattern is stronger here.

Both k and ke mean the same thing here: to, toward.

Ke is used for easier pronunciation before some consonant combinations. Since dveřím begins with dv-, ke dveřím sounds smoother than k dveřím.

So this is mainly a pronunciation-based variant:

• ke dveřím = natural
• k dveřím = possible in theory, but less natural

Můj means my.

• můj pes = my dog

Czech often allows possessive words to be left out when the meaning is obvious, but here můj is included because the speaker is specifically talking about their dog.

Also, můj agrees with pes:

• pes is masculine singular animate
• so the correct nominative form is můj

Yes, Czech word order is more flexible than English, but the given order is very natural.

• můj pes hned běží ke dveřím

This sounds neutral and clear:

• můj pes = topic/subject
• hned = adverb
• běží = verb
• ke dveřím = direction

You could change the word order for emphasis, for example:

• Když zazvoní zvonek u dveří, hned můj pes běží ke dveřím.
• Když zazvoní zvonek u dveří, ke dveřím můj pes hned běží.

But those versions sound more marked. The original sentence is the most straightforward.

Yes. In natural use, it most likely describes a habitual repeated situation:

• Whenever the doorbell rings, my dog immediately runs to the door.

Even though zazvoní is perfective and refers to a single ringing event each time, the whole sentence can still describe something that happens regularly.

So the pattern is:

• each time the bell rings once,
• the dog runs to the door.

You could in some contexts, but běží specifically means runs.

So the best translation is:

• runs to the door

If you said goes to the door, you would lose the idea that the dog moves quickly.

That quick movement also fits well with hned = immediately.

Here are the key dictionary forms:

• když = when
• zazvoní → infinitive zazvonit = to ring, to go off
• zvonek = bell, doorbell
• u = by, at, near
• dveří / dveřím → dictionary form dveře = door
• můj = my
• pes = dog
• hned = immediately, right away
• běží → infinitive běžet = to run
• ke = to, toward

This is a good sentence for seeing how dictionary forms change once cases and conjugation are involved.