Learn Hardcore Czech: Nejčastěji se dívám na film na internetu. - Most often I watch a film on the internet.

dívám se na film can mean “I watch a/the film” in a specific context,
but in a sentence with nejčastěji, it can also mean “I (tend to) watch movies (as an activity).”

So:

Nejčastěji se dívám na film na internetu.
= The thing I most often do is watch a/the film (i.e., watch movies) on the internet.

You can also say na filmy (plural):

Nejčastěji se dívám na filmy na internetu.
This sounds a bit more like “I usually watch (various) films on the internet” and makes the plurality explicit, but both are natural.

Can I say Nejčastěji se dívám na filmy na internetu instead? Does it change the meaning?

Yes, that sentence is perfectly correct.

Nuance:

na film – can sound a bit more generic or “one film at a time,” referring to the activity of watching movies.
na filmy – highlights that there are multiple films in general.

In practice, both can translate as “I most often watch films/movies on the internet,” and the difference is small.

Why is it na internetu and not something like v internetu?

This is mostly idiomatic choice of preposition:

For being or doing something on the internet, Czech uses na:
- na internetu – on the internet
- na Facebooku, na YouTube, na Instagramu

V internetu would sound wrong in normal speech; you’ll basically always say na internetu for “on the internet.”

Why does internet change to internetu in na internetu?

That’s the locative case. After na in the sense of location, Czech typically uses the locative:

kde? (where?) – na čem? → na internetu
base form (nominative): internet
locative singular: internetu

Similarly:

stůl → na stole (on the table)
náměstí → na náměstí (on/in the square)

So na internetu literally means “on the internet” with internet in the locative case.

Why is film in the basic form film, not filmu, in na film?

With dívat se na něco, na takes the accusative (not locative):

na co? – na film (accusative)

For masculine inanimate nouns like film, the nominative and accusative singular look the same:

nominative sg: film
accusative sg: film

So na film is actually accusative, even though it looks like the dictionary form.
Compare with na internetu, where na is locative (location), not accusative (direction/object of looking).

Can I change the word order to Dívám se nejčastěji na film na internetu?

Yes, that’s possible and correct:

Nejčastěji se dívám na film na internetu.
Dívám se nejčastěji na film na internetu.

Both are natural. The difference is minor:

Starting with Nejčastěji puts strong emphasis on frequency (“Most often, what I do is…”).
Putting nejčastěji after the verb sounds a bit more neutral in spoken language.

One important detail: se is a so‑called clitic and tends to appear in the “second position” of the clause, so:

Nejčastěji se dívám... (after the first stressed word Nejčastěji)
Dívám se nejčastěji... (after the verb Dívám)

You wouldn’t usually say Nejčastěji dívám se...; native speakers almost always keep se in that early clitic position.

What exactly does nejčastěji mean, and how is it formed?

Nejčastěji means most often, most frequently.

It comes from the adverb často (often):

často – often
častěji – more often (comparative)
nejčastěji – most often (superlative)

Formed as:

adjective častý – frequent
adverb často – often
comparative adverb častěji – more often
superlative adverb nejčastěji – most often

So Nejčastěji se dívám... = “The thing I do most often is watch…”

What is the difference between dívám se, podívám se, and koukám (se)?

They’re related but not identical:

dívat se (imperfective)
- “to watch / be watching”
- focuses on the ongoing activity or habit
- Nejčastěji se dívám na film... – I most often (tend to) watch films...
podívat se (perfective)
- usually “to take a look / to have a look / to watch once, as a single act”
- Podívám se na ten film večer. – I’ll watch that film this evening (one occasion).
koukat (se) – colloquial, informal
- like “to look, to watch,” more casual
- Nejčastěji koukám na filmy na internetu. – Very informal; same meaning but more colloquial.

In your sentence, dívám se is the standard neutral choice for a habitual action.

Why is there no já in the sentence? Can I say Já se nejčastěji dívám na film na internetu?

Czech usually drops subject pronouns because the verb ending already shows the person:

dívám = I watch (1st person singular), so já is not needed.

So:

Nejčastěji se dívám na film na internetu. – normal, natural sentence

You can add já:

Já se nejčastěji dívám na film na internetu.

But then já is emphasized, often contrasting with someone else:

Já se nejčastěji dívám na film na internetu, ale bratr chodí do kina.
“I most often watch films on the internet, but my brother goes to the cinema.”

How would I make this sentence negative?

You negate the verb dívám se by putting ne- in front of the verb:

Nejčastěji se dívám na film na internetu.
Nejčastěji se na film na internetu nedívám. – “I don’t usually watch films on the internet most often.”

More natural negations for everyday use:

Nejčastěji se na film na internetu nedívám, raději chodím do kina.
“Watching films on the internet is not what I do most; I prefer going to the cinema.”

Note: ne attaches directly to the verb (nedívám se), but because se is a clitic, the usual order in a simple clause is ne + verb + se:

Nedívám se na film na internetu. – I don’t watch films on the internet.

Does the Czech present tense here mean “I am watching” right now or “I usually watch”?

In this sentence, with nejčastěji, the present tense clearly expresses a habitual action:

Nejčastěji se dívám na film na internetu.
= “I most often / usually watch a film on the internet.”

Czech present tense is used for:

actions happening now:
- Dívám se na film. – I am watching a film (right now).
regular or habitual actions:
- Každý večer se dívám na film. – I watch a film every evening.

Here, the adverb nejčastěji forces the habitual reading.