Learn Hardcore German: Manchmal ist Grammatiklernen langweilig, aber es gehört einfach zum Lernen. - Sometimes learning grammar is boring, but it simply belongs to learning.

It’s capitalized because in German all nouns are capitalized, and Grammatiklernen is functioning as a noun here (an activity, “the learning of grammar”).

Is Grammatiklernen a verb or a noun? It looks like an infinitive.

It comes from the infinitive verb lernen, but in this sentence it is used as a noun.

German often turns infinitives into nouns to talk about activities in a general way:

lesen (to read) → das Lesen (reading)
schreiben (to write) → das Schreiben (writing)
Grammatik lernen (to learn grammar) → Grammatiklernen (learning grammar)

So here Grammatiklernen is a nominalized infinitive: the verb form used as a noun.

Why is there no article, like das Grammatiklernen?

You could say das Grammatiklernen, and it’s grammatically correct, but:

In general statements about activities, German often drops the article:
- Rauchen ist ungesund. = Smoking is unhealthy.
- Lesen macht Spaß. = Reading is fun.
- Grammatiklernen ist langweilig. = Learning grammar is boring.

Without the article, it sounds more like a general statement about the activity as a whole, not about a specific instance of it.

Why does the sentence start with Manchmal instead of the subject? Could I say Grammatiklernen ist manchmal langweilig?

Both are correct:

Manchmal ist Grammatiklernen langweilig.
Grammatiklernen ist manchmal langweilig.

German main clauses follow the “verb in second position” rule. The element in first position can be:

the subject: Grammatiklernen ist manchmal langweilig.
a time adverb like Manchmal: Manchmal ist Grammatiklernen langweilig.

Putting Manchmal first emphasizes “sometimes” a bit more: it highlights the time aspect.

Why is it just langweilig and not something like langweiliges Grammatiklernen?

There are two different uses of adjectives in German:

Predicative adjective (after sein, werden, bleiben):
- Grammatiklernen ist langweilig.
  Here langweilig describes the state of the subject. It does not take an ending.
Attributive adjective (directly before a noun):
- langweiliges Grammatiklernen
  Here langweiliges would need an ending, because it sits in front of the noun and works like part of the noun phrase.

In the sentence you gave, langweilig is predicative (after ist), so it has no ending.

What kind of conjunction is aber, and why is the word order aber es gehört and not aber gehört es?

Aber is a coordinating conjunction (like und, oder, denn). Coordinating conjunctions:

link two main clauses of equal status, and
do not change the normal word order of the second clause.

So after aber, the next clause is just a normal main clause:
aber es gehört einfach zum Lernen.
(Subject es, verb gehört in second position.)

A structure like aber gehört es would put the verb first and would normally be used only in special cases (e.g. in questions or for strong emphasis), not in this neutral statement.

What does es refer to in aber es gehört einfach zum Lernen?

Es is a pronoun referring back to the idea in the first clause, namely Grammatiklernen (learning grammar) – or more generally, the fact that learning grammar can be boring.

So the sense is:

Manchmal ist Grammatiklernen langweilig, aber es gehört einfach zum Lernen.
→ Sometimes learning grammar is boring, but it (learning grammar) is simply part of learning.

What does the verb gehören mean in this context, and why is it used with zu?

The verb gehören has two main uses:

gehören + Dativ → to belong to (in the sense of ownership)
- Das Buch gehört mir. = The book belongs to me.
zu etwas gehören → to be part of something / to belong to a group or whole
- Fehler machen gehört zum Lernen. = Making mistakes is part of learning.
- Grammatiklernen gehört zum Lernen. = Learning grammar is part of learning.

In your sentence, it’s the second meaning:
gehören zu = “to be part of”.

What exactly is zum in zum Lernen?

Zum is a contraction of:

zu (preposition) + dem (dative article “the”)
→ zu dem → zum

So literally:
gehört zum Lernen = “belongs to the learning”

Because zu always takes the dative, you get zu dem Lernen → zum Lernen.

Why is Lernen capitalized in zum Lernen?

Again, this is a nominalized infinitive:

lernen (to learn) → das Lernen (learning)

When a verb is turned into a noun (especially after a preposition), it is:

capitalized, and
treated grammatically like a neuter noun.

After zu, which takes the dative, you get:

zu dem Lernen → zum Lernen

So Lernen is written with a capital L because it is functioning as a noun here.

What does einfach mean here? Is it “simple” or “simply”?

In this sentence, einfach means “simply” / “just”, not “easy”.

aber es gehört einfach zum Lernen.
≈ “but it is simply part of learning” / “but it just goes with learning”.

When einfach is used:

before a noun: it often means “simple” or “plain”
- ein einfaches Problem = a simple problem
as an adverb (as here): it often means “simply / just”, often softening or emphasizing a statement
- Das ist einfach so. = That’s just the way it is.

Could I say aber es ist einfach Teil des Lernens instead of aber es gehört einfach zum Lernen? Is there a difference?

Yes, you can say:

Manchmal ist Grammatiklernen langweilig, aber es ist einfach Teil des Lernens.

This is grammatically correct and very natural. The differences are subtle:

gehört zum Lernen
- uses the idiomatic verb gehören zu = “to be part of / to belong to”
- slightly more idiomatic and common in phrases like
  - Fehler machen gehört zum Lernen.
ist Teil des Lernens
- more literal: “is part of learning”
- slightly more formal or neutral, but still very normal in everyday speech.

Both mean essentially the same thing. The original version with gehören zum Lernen sounds very natural and a bit more idiomatic.