Learn Hardcore Ukrainian: Уночі небезпечно бігати в парку. - At night it is dangerous to run in the park.

The base noun is ніч (night).
In the locative case (used with prepositions like в/у to mean in/at), it becomes ночі.
The preposition у
- ночі has fused into one adverb уночі over time.

So:

уночі = at night (adverb)
уночі answers the question “коли?” (when?).

Can I also say Вночі instead of Уночі? Is there any difference?

Yes, Уночі and Вночі are both correct and both mean “at night.”

Ukrainian often alternates у and в for reasons of euphony (to make pronunciation smoother):

After a consonant, speakers often prefer у to avoid consonant clusters.
After a vowel, в often sounds smoother.

In practice:

Уночі небезпечно бігати в парку.
Вночі небезпечно бігати в парку.

Both are acceptable in modern Ukrainian. Уночі is a bit more common in writing, but this is not a strict rule.

Why is there no word for “it” in the sentence? How do you say “It is dangerous to run in the park at night” without “it”?

Ukrainian often uses impersonal sentences where English uses “it.”

Structure here:

небезпечно – an adverb: dangerously / it is dangerous
бігати – infinitive: to run
No explicit subject.

So Уночі небезпечно бігати в парку. literally is something like:

“At night, (it is) dangerous to run in the park.”
(The verb “is” and the pronoun “it” are just implied.)

This impersonal pattern is very common:

Тут холодно. – It is cold here.
Жити в цьому місті дорого. – It is expensive to live in this city.
Небезпечно переходити дорогу тут. – It is dangerous to cross the road here.

Ukrainian simply doesn’t need a dummy subject like “it.”

What part of speech is небезпечно here, and how is it related to небезпечний?

Небезпечно here is an adverb formed from the adjective небезпечний (dangerous).

небезпечний – dangerous (adjective, masculine)
- небезпечна (fem.)
- небезпечне (neut.)
- небезпечні (plural)
небезпечно – dangerously / it is dangerous (adverb)

Compare:

Небезпечний парк. – A dangerous park. (adjective modifying a noun)
Уночі небезпечно бігати. – At night, it is dangerous to run. (adverb modifying the whole situation)

So in this sentence, небезпечно functions like English “it is dangerous.”

Why is the verb бігати used, and not бігти or some other form of to run?

Ukrainian has two main verbs for “to run”:

бігати – multidirectional / repeated / general running
- Used for:
  - habitual actions: Я люблю бігати. – I like running.
  - general ability/possibility: Тут можна бігати. – One can run here.
бігти – unidirectional, in one specific direction, usually a single movement
- Я біжу додому. – I am running home (now, in one direction).

In Уночі небезпечно бігати в парку, the idea is running in general in the park at night (not one specific run, one direction), so бігати is the natural choice.

If you said Уночі небезпечно бігти в парк, it would mean more specifically:

“At night it’s dangerous to run (once, in one direction) to the park.” which is a different nuance.

What case is в парку, and why is парк changing to парку?

В парку uses the locative case (also called the prepositional case in some traditions).

Nominative (dictionary form): парк – park
Locative singular: у парку / в парку – in the park

The locative is used after в/у, на, etc., when you mean location:

в парку – in the park
у місті – in the city
на столі – on the table

So бігати в парку means “to run in the park.”
The ending -у on парку is the regular locative singular ending for masculine hard-stem nouns like парк.

Could I say бігати у парку instead of бігати в парку?

Yes, you can say бігати у парку. Both в парку and у парку are grammatically correct.

Again, it’s about euphony (sound):

After a consonant, speakers often prefer у to avoid [consonant + в + consonant] clusters.
After a vowel, в can be smoother.

In your sentence:

бігати в парку – ends with the vowel sound -и, then в, then п; this is easy to say.
бігати у парку – also fine and sometimes sounds slightly more fluid in fast speech.

Native speakers freely alternate в and у in many contexts. Both versions are natural.

Can I change the word order? For example: Небезпечно в парку бігати уночі?

Yes, Ukrainian word order is relatively flexible, especially in simple sentences like this. Changing word order usually changes emphasis, not basic meaning.

Some natural variants:

Уночі небезпечно бігати в парку.
(Neutral; sets time first.)
Небезпечно бігати в парку уночі.
(Neutral; starts with the idea “it’s dangerous.”)
В парку небезпечно бігати уночі.
(Emphasizes in the park as the place where it’s dangerous.)

Your version:

Небезпечно в парку бігати уночі. – acceptable, but sounds a bit more “spoken” and slightly less neutral in style.

All of them are understandable; the original word order is the most straightforward and neutral.

How would I turn this into a question: “Is it dangerous to run in the park at night?”

You keep the same words and just use question intonation (rising pitch at the end):

Уночі небезпечно бігати в парку? – Is it dangerous to run in the park at night?

Ukrainian often forms yes/no questions without changing word order, just by intonation.

If you want it explicitly marked as a question in writing, you can add чи:

Чи небезпечно бігати в парку уночі?

Both are correct; without чи is more common in everyday speech.

How would I say “It’s dangerous for children to run in the park at night” in Ukrainian?

You keep the same structure and add “for children” in the dative case:

Уночі небезпечно дітям бігати в парку.

Breakdown:

діти – children (nominative plural)
дітям – to/for children (dative plural)

So the pattern is:

(Кому?) дітям небезпечно (що робити?) бігати.
– It is dangerous for children to run.