Learn Hardcore Czech: Když je noc a já nemohu spát, dívám se na film na internetu. - When it is night and I cannot sleep, I watch a film on the internet.

Když je noc literally means “when it is night” – you’re describing the time of day with a simple nominative noun (noc).
Když je v noci would mean “when it is at night”, which sounds odd in Czech in this kind of sentence; you normally say je noc, not je v noci.
nocí is the instrumental case of noc and is used in expressions like šel nocí (he walked through the night). Saying je nocí is not natural standard Czech for “it is night.”

So Když je noc is the normal, idiomatic way to say “When it is night”.

What is the difference between když and kdy? Why is když used here?

když is a conjunction meaning “when” (or sometimes “whenever”) that introduces a subordinate clause:
- Když je noc, dívám se na film. – When it is night, I watch a movie.
kdy is an interrogative adverb, used mainly in questions:
- Kdy je noc? – When is night?
- Kdy se díváš na film? – When do you watch a movie?

Because the clause Když je noc a já nemohu spát is not a question but a “when”-clause, když is required, not kdy.

Can I replace když with až in this sentence?

Not naturally.

když = “when / whenever” in a general, habitual, or neutral sense.
až often means “when (in the future, at some later time)” or “only when”.

Your sentence is about a general habit:

Když je noc a já nemohu spát, dívám se na film…
= Whenever it’s night and I can’t sleep, I watch a movie…

Using až (Až je noc…) would sound wrong here, because it suggests a single future point (“when at some later point it becomes night”), not a regular pattern.

Why is it já nemohu spát and not just nemohu spát?

Both are possible:

Nemohu spát – neutral, and usually the most natural.
Já nemohu spát – adds emphasis to I: I can’t sleep (as opposed to someone else), or just sounds a bit more personal / expressive.

In spoken Czech, people often drop já when the subject is clear from verb endings, but adding já is not wrong; it just slightly emphasizes the subject.

What is the difference between nemohu and nemůžu?

They mean the same thing: “I cannot / can’t”.

nemohu – more formal, standard, bookish style.
nemůžu – more colloquial, very common in everyday speech.

In normal conversation, you’ll hear nemůžu spát much more often.
In written, formal, or carefully standard language, nemohu spát is completely fine and maybe slightly more “correct” or neutral.

Why is it nemohu spát and not nemohu spím?

You need the infinitive after moci / nemoci (mohu / nemohu):

mohu / nemohu + infinitive
- Nemohu spát. – I can’t sleep.
- Mohu přijít. – I can come.

Spát is the infinitive (to sleep).
Spím is the 1st person singular present (I sleep). You cannot say nemohu spím; that would be ungrammatical.

Why is dívám se reflexive? Why not just dívám?

In Czech, the verb dívat se is reflexive by nature and means “to look at / to watch”:

dívat se na něco – to look at / watch something
- Dívám se na film. – I’m watching a movie.

Without se, dívat has a different meaning (it’s used transitively in some fixed expressions, but not in the sense of “watch a movie”). So:

You must say dívám se for I watch / I am watching.
dívám alone (without se) is not used with this meaning.

Why is it dívám se na film and not just dívám se film?

The verb dívat se requires the preposition na before its object:

dívat se na + accusative – to look at / watch something
- Dívám se na film. – I’m watching a movie.
- Dívám se na televizi. – I’m watching TV.
- Dívám se na tebe. – I’m looking at you.

Saying dívám se film is incorrect; the preposition na is obligatory here.

Why do we say dívám se and not podívám se in this sentence?

This is about aspect in Czech verbs:

dívat se – imperfective, focuses on ongoing or repeated action (“to be watching,” “to watch in general”).
podívat se – perfective, focuses on a single, completed act (“to have a look (once),” “to watch once / check something out”).

Your sentence describes a habitual action – what you typically do when you can’t sleep. For habits and repeated actions, Czech prefers the imperfective:

Když je noc a já nemohu spát, dívám se na film… – general habit.

If you said podívám se, it would sound more like a one-time decision:

When it’s night and I can’t sleep, I will (then) watch a movie (once).

Why is it na internetu and not v internetu?

In modern Czech, na internetu (on the internet) is the standard phrase.

na internetu – idiomatic, means “on the internet / online.”
v internetu is technically possible in very specific technical contexts (e.g. inside a network system), but for everyday usage it sounds unnatural or wrong.

So with normal activities, you say:

Na internetu hledám informace. – I look for information on the internet.
Dívám se na film na internetu. – I watch a movie on the internet.

Why is there a comma before dívám se na film na internetu?

The comma separates the subordinate clause from the main clause:

Když je noc a já nemohu spát, – subordinate “when”-clause
dívám se na film na internetu. – main clause

In Czech, you generally must put a comma between a když-clause and the main clause, no matter which one comes first:

Dívám se na film na internetu, když je noc a nemohu spát. – comma also needed here.

Why is there no article before film in na film?

Czech has no articles (no equivalent of English a / an / the).

na film can mean “(a) film”, “(the) film”, or just “movies” in general, depending on context.
The definiteness or indefiniteness is usually understood from context, not from a word form.

So dívám se na film can be translated as:

I watch a movie,
I watch the movie,
or I watch movies (if we mean “some movie, not specified”).

Is the word order fixed? Could I say Dívám se na film na internetu, když je noc a nemohu spát?

Yes, you can change the order; Czech word order is more flexible than English, though there are preferences.

Both are correct:

Když je noc a já nemohu spát, dívám se na film na internetu.
Dívám se na film na internetu, když je noc a já nemohu spát.

The main difference is what you present first:

Starting with Když je noc… emphasizes the condition / time.
Starting with Dívám se na film… emphasizes the activity first, then explains when you do it.

Grammatically, both are fine; the comma stays in either position.