Learn Hardcore German: Im Lerntagebuch notiere ich, was ich heute verstanden habe und was noch schwer ist. - In the study diary I note what I understood today and what is still difficult.

We use dem (dative) after in here because the meaning is location (“in the diary”) rather than movement (“into the diary”). Location after in takes the dative case.

Why is it Lerntagebuch all in one word, and why is it capitalized?

German loves compound nouns. Lerntagebuch = Lern- (learning) + Tagebuch (diary).

So it literally means “learning diary”.
In German, nouns are always written with a capital first letter: Lerntagebuch.
Compounds are usually written as one word, not separated by spaces or hyphens (unless they get very long or for clarity).

Grammatically:

The base noun is das Tagebuch (neuter).
The whole compound keeps that gender: das Lerntagebuch.
In the sentence we see it in the dative singular: im Lerntagebuch = in dem Lerntagebuch.

Why is the word order Im Lerntagebuch notiere ich and not Ich notiere im Lerntagebuch?

Both are correct; they just emphasize different parts.

German main clauses follow the verb-second (V2) rule:

Exactly one element comes first (the “first position”),
Then the conjugated verb,
Then the subject or other elements.

Two options:

Ich notiere im Lerntagebuch …
- First position: Ich
- Verb (2nd): notiere
  → Neutral, very common order.
Im Lerntagebuch notiere ich …
- First position: Im Lerntagebuch
- Verb (2nd): notiere
- Subject comes after: ich

The second version puts a bit more focus on where you are writing: In the learning diary, I note down …. It’s a typical stylistic variation in German.

What exactly is was doing in was ich heute verstanden habe? Isn’t was just a question word?

Here was is not a question word. It introduces a content/relative clause meaning “what” in the sense of “the things that …”.

English: I write down *what I understood today.*
German: Ich notiere, was ich heute verstanden habe.

In this use:

was = a kind of relative pronoun meaning “what (things)”.
It introduces a subordinate clause and refers to an unspecified thing or set of things.

You can think of it as standing for das, was:

Ich notiere das, was ich heute verstanden habe.
→ I write down that which I understood today.

Why is the verb at the end in was ich heute verstanden habe and was noch schwer ist?

Because these are subordinate clauses (Nebensätze).

Rules:

In main clauses: conjugated verb is usually in second position.
In subordinate clauses introduced by was, dass, weil, wenn, etc., the conjugated verb goes to the end.

So:

Main clause: Ich habe das heute verstanden.
(Verb 2nd: habe)
Subordinate clause: …, was ich heute verstanden habe.
(Conjugated verb habe at the end)

Similarly:

Main clause: Das ist noch schwer.
Subordinate clause: …, was noch schwer ist.

Both habe and ist are at the end because was starts subordinate clauses.

Why is it verstanden habe and not habe verstanden in the clause?

In the perfect tense in German, you have:

An auxiliary verb (haben or sein, conjugated)
A past participle (verstanden)

Word order:

In a main clause:
Ich habe das heute verstanden.
Auxiliary (2nd) + participle at (or near) the end.
In a subordinate clause:
…, was ich heute verstanden habe.
Here the rule is: both parts of the verb go to the end, with the participle before the conjugated auxiliary.
→ … verstanden habe

So habe is last because it’s the conjugated verb in a subordinate clause.

Why does heute come before verstanden and not after it?

In subordinate clauses with a compound verb like verstanden habe, the typical order is:

Other elements (subject, adverbs, objects)
Past participle
Conjugated auxiliary verb

So:

… was ich heute verstanden habe.
Subject: ich
Time adverb: heute
Participle: verstanden
Auxiliary: habe

Ich heute verstanden habe is the natural word order.
Variations like … was ich verstanden heute habe sound wrong or at least very unnatural.

What does noch mean in was noch schwer ist, and what nuance does it add?

Noch here means “still” (or “yet”).

was schwer ist = what is difficult
was noch schwer ist = what is still difficult / what is still hard for me

The nuance:

It implies that some things have already become easier/understood, but these things are not yet easy. There has been some progress, but these parts remain difficult for now.

What is the difference between schwer and schwierig here?

Both can often mean “difficult”, but there are tendencies:

schwer
- Literally “heavy”
- Often more emotional / subjective: hard for me, tough, burdensome
- Can refer to physical weight or difficulty.
schwierig
- More objective / technical: complicated, problematic, tricky to solve.

In this sentence, you could also say:

… und was noch schwierig ist.

That would be understood.
schwer sounds slightly more informal and personal (“what is (still) hard for me”), which fits a learning diary well.

Why is there a comma before und in …, was ich heute verstanden habe und was noch schwer ist?

The comma after ich in the original is:

Im Lerntagebuch notiere ich, was ich heute verstanden habe und was noch schwer ist.

This comma is required because it introduces the first subordinate clause:

Main clause: Im Lerntagebuch notiere ich
Subordinate clauses (objects of notieren):
- was ich heute verstanden habe
- (und) was noch schwer ist

was ich heute verstanden habe und was noch schwer ist is treated as one complex subordinate clause with two coordinated parts.

So:

Comma before was (not before und) because a Nebensatz (subordinate clause) begins.
und just links the two was… parts inside that subordinate structure.

You could also write more explicitly:

…, was ich heute verstanden habe, und was noch schwer ist.
But in modern usage, the single comma before the entire was … und was … block is very common and acceptable.

Could I also say Ich notiere im Lerntagebuch, was ich heute verstanden habe …? Is that different?

Yes, that’s perfectly correct:

Ich notiere im Lerntagebuch, was ich heute verstanden habe und was noch schwer ist.

Difference is mainly word order and emphasis:

Im Lerntagebuch notiere ich, …
- Emphasis on “In the learning diary …”
- Style: slightly more varied, less “Ich, ich, ich”.
Ich notiere im Lerntagebuch, …
- More neutral, typical subject-first order.
- Emphasis on “I note down …”.

In terms of meaning, they are the same.

What is the object of notiere in this sentence?

The verb notieren (to note down) needs something that is being noted. In this sentence, that “something” is not a simple noun like “the answers”; it’s expressed by subordinate clauses:

was ich heute verstanden habe
und was noch schwer ist

Together they function as the direct object of notiere:

Ich notiere [was ich heute verstanden habe und was noch schwer ist].

In English:
I note down [what I understood today and what is still difficult]. The bracketed part is the object of “note down”.