Heute Abend läuft ein interessanter Film im Fernsehen.

In German, all nouns are capitalized, and Abend is a noun meaning evening.

In heute Abend, German uses a structure that literally looks like today evening, but it means this evening / tonight. Even though Abend functions a bit like part of a time expression here, it is still a noun, so it stays capitalized.

German often puts time expressions near the beginning of the sentence, especially when setting the scene.

So:

Heute Abend läuft ein interessanter Film im Fernsehen.

starts with Heute Abend to mean Tonight, ...

This is very natural in German. You could also change the order, but then the verb still has to stay in the second position:

Ein interessanter Film läuft heute Abend im Fernsehen.

Both are correct, but the first version emphasizes when it happens.

In a normal German main clause, the finite verb usually comes in second position. This is one of the most important word-order rules in German.

In this sentence:

Heute Abend = first position
läuft = second position

Then the rest follows:

ein interessanter Film im Fernsehen

Even though Heute Abend contains two words, it counts as one sentence element. So the verb still comes second.

Yes, laufen literally means to run, but German often uses it idiomatically for things that are showing, playing, or being broadcast.

So with films, shows, or programs, laufen can mean:

• to be on
• to be showing
• to be airing

So:

Ein Film läuft im Fernsehen
means
A film is on TV

This is a very common usage.

Because ein interessanter Film is the subject of the sentence, so it is in the nominative case.

Ask yourself: What is on TV tonight?
Answer: ein interessanter Film

That makes it the subject.

Masculine nominative with ein takes:

• ein interessanter Film

If it were a direct object, then you would get the accusative:

• Ich sehe einen interessanten Film.
  = I am watching an interesting film.

So here, ein interessanter Film is nominative, not accusative.

German adjectives usually take an ending when they come before a noun.

Here, interessant becomes interessanter because it is describing the masculine noun Film in the nominative singular after ein.

So:

• ein interessanter Film
• ein guter Film
• ein neuer Film

This is part of the German adjective ending system. In this case, -er shows masculine nominative singular.

im is a contraction of in dem:

• in dem → im

So im Fernsehen literally looks like in the television, but in natural English it means:

• on television
• on TV

German often uses in where English uses on for media expressions like this.

Because Fernsehen here is treated as a noun, and after in in this expression, German uses the dative case.

So:

• in dem Fernsehen → im Fernsehen

This is a fixed, very common expression meaning on TV / on television.

Also, Fernsehen is neuter, so its dative singular form with the definite article is dem.

Here it is a noun, which is why it is capitalized: Fernsehen.

Compare:

• fernsehen = to watch TV or to watch television
• das Fernsehen = television / TV as a noun

In this sentence, im Fernsehen uses the noun form.

It is understandable, but läuft is much more natural here.

German usually says a film or program läuft on TV, not ist on TV, when talking about something being broadcast.

So the most idiomatic version is:

Heute Abend läuft ein interessanter Film im Fernsehen.

Using ist would sound less natural to many native speakers.

Yes, German word order is flexible, as long as the finite verb stays in second position in a main clause.

For example:

Ein interessanter Film läuft heute Abend im Fernsehen.
Im Fernsehen läuft heute Abend ein interessanter Film.

These all mean roughly the same thing, but the emphasis changes:

• Heute Abend ... emphasizes the time
• Ein interessanter Film ... emphasizes what is on
• Im Fernsehen ... emphasizes where it is showing

Yes, Film is masculine in German:

• der Film
• ein Film

That is why you get masculine forms like:

• ein interessanter Film
• der interessante Film

Knowing the gender of the noun is important because it affects:

• the article
• adjective endings
• pronouns later on

Both can work, depending on context.

• Heute Abend often corresponds to this evening
• In many contexts, especially casual ones, English would simply say tonight

So this sentence could naturally match either of these ideas:

• This evening, an interesting film is on TV.
• Tonight, an interesting film is on TV.

The exact English choice depends on style, but the German phrase itself is very common and straightforward.