Learn Hardcore Ukrainian: Коли мені нудно, я дивлюся серіал.

мені нудно – neutral, most common way to say I’m bored.
я нудний – means I am boring (as a person, not that I feel bored).
я нудьгую – can mean I’m bored or I’m missing someone, but in everyday speech people usually say мені нудно for I’m bored (with nothing to do) and use нудьгувати за кимось (to miss someone):
- Я нудьгую за тобою. – I miss you.

So in this sentence, мені нудно is exactly the natural way to say when I’m bored.

What case is мені, and what does it literally mean here?

Мені is the dative singular of я (I).

я – I (nominative)
мені – to me / for me (dative)

In this construction, the dative marks the experiencer of a feeling or state:

мені нудно – I’m bored (literally: to me it is boring)
мені холодно – I’m cold
мені сумно – I’m sad

So мені doesn’t mean “I” as the grammatical subject; it marks the person who experiences the state described by нудно.

Is нудно “bored” or “boring”? How is it different from нудний?

Нудно is a predicative adverb describing a state:

мені нудно – I am bored / I feel bored

Нудний is an adjective meaning boring (describing a thing or person):

Цей серіал нудний. – This series is boring.

So:

To say I’m bored: мені нудно
To say It’s boring: це нудно or це нудний серіал

Can you drop мені and just say Коли нудно, я дивлюся серіал?

Yes, you can say:

Коли нудно, я дивлюся серіал.

This sounds a bit more general/impersonal, like “When (one) is bored / when it’s boring, I watch a series.”

With мені you clearly say it’s you who is bored:

Коли мені нудно, я дивлюся серіал. – the most typical version.

Both are grammatically correct; мені нудно is just more explicit about the experiencer.

What is the verb дивлюся? What is its infinitive and how is it conjugated?

Дивлюся is the 1st person singular, present tense of дивитися (to watch, to look at).

Infinitive: диви́тися – to watch
1st sg: я ди́влюся – I watch / I am watching
2nd sg: ти ди́вишся – you watch
3rd sg: він/вона/воно ди́виться – he/she/it watches
1st pl: ми ди́вимося – we watch
2nd pl: ви ди́витеся – you (pl/formal) watch
3rd pl: вони ди́вляться – they watch

The -ся ending is part of the verb; many Ukrainian verbs of watching, washing, moving, etc. take this reflexive-like ending, even when they are not reflexive in meaning.

In your sentence:

я дивлюся серіал = I watch a (TV) series or I’m watching a series (here: a habitual action).

What’s the difference between дивлюся and дивлюсь?

They are the same verb form with two spellings:

дивлюся – full form
дивлюсь – shortened form (the я sound in -ся is dropped)

Meaning: identical – I watch / I am watching.

Usage:

In speech, both are very common; дивлюсь sounds a bit more colloquial.
In careful or formal writing, дивлюся is usually preferred, but дивлюсь is not wrong.

So you can say:

Коли мені нудно, я дивлюся серіал.
Коли мені нудно, я дивлюсь серіал.

Both are correct.

Can the word order be changed, e.g. Я дивлюся серіал, коли мені нудно?

Yes. Both are normal:

Коли мені нудно, я дивлюся серіал.
Я дивлюся серіал, коли мені нудно.

The meaning is the same: When I’m bored, I watch a series.

The difference is only in emphasis:

Starting with коли мені нудно emphasizes the condition (whenever I’m bored…).
Starting with я дивлюся серіал emphasizes the action (I watch a series when…).

Inside the small clause, мені нудно is the normal order; нудно мені is possible but more emotional/poetic.

Why is there a comma in Коли мені нудно, я дивлюся серіал?

Because Ukrainian has two clauses here, linked by the conjunction коли (when):

Коли мені нудно – subordinate time clause (when I’m bored)
я дивлюся серіал – main clause (I watch a series)

In Ukrainian, a subordinate clause introduced by words like коли, що, якщо is separated by a comma, whether it comes before or after the main clause:

Коли мені нудно, я дивлюся серіал.
Я дивлюся серіал, коли мені нудно.

Both need a comma.

What exactly does серіал mean? Is it the same as “series” or “soap opera”?

Серіа́л is a TV (or online) series – a show made of episodes that continue the same story or characters.

It’s close to English “series” / “TV show” (especially scripted shows). It is not limited to soap operas:

серіал – series / TV show
серія – an episode
мильна опера – soap opera (literally “soap opera”)

Examples:

Я дивлюся український серіал. – I’m watching a Ukrainian series.
Це дуже популярний серіал на Netflix. – This is a very popular series on Netflix.

Is the sentence about something I do right now, or about a general habit?

In this form, it most naturally expresses a general habit:

Коли мені нудно, я дивлюся серіал.
= Whenever I’m bored, I (usually) watch a series.

The present tense of дивитися here is like English simple present for regular actions.

For a specific future situation, you might say:

Коли мені буде нудно, я подивлюся серіал.
- буде нудно – will be bored (future of to be)
- подивлюся – perfective I will watch (once, one time)

How would you say “What do you watch when you’re bored?” using the same structure?

You can keep the same pattern коли тобі нудно (when you are bored) and ask:

Що ти дивишся, коли тобі нудно? – What do you watch when you’re bored?

Breakdown:

що – what
ти дивишся – you watch (2nd sg)
коли – when
тобі нудно – you (to you) are bored (dative + нудно)

This directly mirrors the original sentence but in question form.

How is the whole sentence pronounced, and where is the stress?

Stressed syllables in bold:

Коли́ мені́ ну́дно, я ди́влюся серіа́л.

Rough guide:

Коли́ – ko-LY
мені́ – me-NEE
ну́дно – NOO-dno
ди́влюся – DYV-lyu-sia
серіа́л – se-ri-AL

Say it smoothly as two chunks:

Коли мені нудно, (small pause) я дивлюся серіал.