Learn Hardcore German: Abends ist das Fernsehen oft langweilig.

Abends here is an adverb meaning “in the evenings / at night in general”, not a full noun phrase.
Adverbs do not take an article or a preposition, so you simply say abends, not am Abend.
Am Abend is also possible, but it usually points to a more specific evening or time frame, while abends sounds more like a general habit.

What is the exact difference between abends and am Abend?

abends = in the evenings (generally, repeatedly, as a habit)
- Abends sehe ich oft fern. – I often watch TV in the evenings.
am Abend = in the evening (more like one particular evening or one time period)
- Am Abend sehe ich heute fern. – I’ll watch TV this evening.

In practice, they can overlap, but abends strongly suggests a regular, repeated situation.

Why is Abends capitalized here? Is it a noun?

The normal form is the adverb abends, which is written with a lowercase a.
In your sentence it appears as Abends only because it is the first word in the sentence, and German always capitalizes the first word.
Grammatically, it is an adverb, not a noun.

What is the subject of this sentence? Is Abends the subject?

The subject is das Fernsehen.
Structure: Abends (time adverbial) – ist (verb) – das Fernsehen (subject in nominative) – oft langweilig (predicate).
So it means literally: In the evenings, the TV is often boring.

Why do we say das Fernsehen and not just Fernsehen without the article?

With the article, das Fernsehen refers to “television as a medium / what is on TV”.
Without the article, Fernsehen is more like the activity “watching TV”.

Compare:

Fernsehen ist abends oft langweilig. – Watching TV is often boring in the evenings.
Das Fernsehen ist abends oft langweilig. – What’s on (on TV) in the evenings is often boring.

Both are grammatically correct; the nuance is a bit different.

What is the difference between das Fernsehen, fernsehen, and der Fernseher?

das Fernsehen (noun, neuter) – television as a medium/institution or the TV programming.
- Das Fernsehen ist abends oft langweilig.
fernsehen (verb) – to watch TV.
- Abends sehe ich fern. – I watch TV in the evenings.
der Fernseher (noun, masculine) – the physical TV set.
- Der Fernseher steht im Wohnzimmer. – The TV set is in the living room.

Why is the verb ist singular here? Could you ever say sind?

Das Fernsehen is a singular neuter noun, so the verb must be ist (3rd person singular).
You would use sind only with a plural subject, for example:

Abends sind die Fernsehsendungen oft langweilig. – The TV shows are often boring in the evenings.

Here, with das Fernsehen, only ist is correct.

Where can the adverb oft go? Are sentences like Abends ist oft das Fernsehen langweilig or …ist das Fernsehen langweilig oft correct?

The neutral and most natural position is:

Abends ist das Fernsehen oft langweilig.
or with a different order:
Das Fernsehen ist abends oft langweilig.

Abends ist oft das Fernsehen langweilig is grammatically possible but sounds marked and focuses on das Fernsehen (as opposed to something else).
…ist das Fernsehen langweilig oft is ungrammatical; frequency adverbs like oft do not go after the predicate adjective here.

Why doesn’t langweilig have an ending, like langweiliges or langweilige?

Langweilig here is a predicate adjective after the verb sein (ist).
Predicate adjectives in German do not take endings:

Das Buch ist langweilig.
Der Film ist spannend.

Adjective endings appear when the adjective stands before a noun (attributive):

ein langweiliges Programm – a boring program
die langweilige Sendung – the boring show

Could I replace oft with other words like häufig or meistens? Would the meaning change?

Yes, and the nuance changes slightly:

oft – often (neutral frequency)
häufig – frequently (similar to oft, sometimes a bit more formal or emphatic)
meistens – mostly, most of the time (stronger: it’s boring more often than not)

So you could say:

Abends ist das Fernsehen häufig langweilig.
Abends ist das Fernsehen meistens langweilig.

All are correct; choose based on how strong you want the statement to be.

Does the present tense ist talk about one specific evening or a general habit?

In this sentence, with abends and oft, the present tense clearly expresses a general habit:
Abends ist das Fernsehen oft langweilig. = TV is often boring in the evenings (in general).

For a specific evening you would use a different time expression, e.g.:

Heute Abend ist das Fernsehen langweilig. – Tonight, TV is boring.

Can I express the same idea in other ways in German?

Yes, for example:

Abends läuft im Fernsehen oft nur Langweiliges.
– In the evenings, there’s often only boring stuff on TV.
Abends sind die Fernsehsendungen oft langweilig.
– The TV shows are often boring in the evenings.
Abends gibt es im Fernsehen oft nichts Interessantes.
– In the evenings, there is often nothing interesting on TV.

All keep the core idea but highlight slightly different aspects (the shows, what’s on, etc.).