Learn Hardcore German: Ich schreibe das Zitat in mein Notizbuch.

Zitat is a neuter noun: das Zitat (the quotation).
In the sentence, das Zitat is the direct object (the thing being written), so it is in the accusative case.
For neuter nouns, nominative and accusative are the same: das.

So:

Nominative: das Zitat (subject)
Accusative: das Zitat (direct object)

You see das, not der or die, because Zitat is neuter and used as a direct object.

How can I tell that das Zitat is in the accusative case here?

Ask yourself: What is being written?

Subject: Ich (I) – the one doing the action.
Verb: schreibe (write).
Direct object: das Zitat – the thing that receives the action of writing.

The direct object in German is in the accusative case. So das Zitat is accusative, even though it looks the same as nominative for neuter nouns. With masculine nouns, the change is more visible, e.g.:

Nominative: der Text
Accusative: den Text

In your sentence, you just have to know that das Zitat is functioning as the direct object, so it is accusative.

Why is it in mein Notizbuch and not in meinem Notizbuch?

The preposition in can take either accusative or dative, depending on the meaning:

Accusative with movement into something (direction):
- Ich schreibe das Zitat in mein Notizbuch.
  → The quote is going into the notebook (you are putting it there).
Dative with position in something (location, no movement):
- Das Zitat steht in meinem Notizbuch.
  → The quote is in the notebook already, you are just describing its location.

In your sentence, there is a clear movement: you are writing the quote into the notebook. That is why in takes the accusative case, and mein Notizbuch is accusative:

Neuter, accusative singular → mein Notizbuch, not meinem Notizbuch.

Could I say in das Notizbuch or ins Notizbuch instead of in mein Notizbuch?

Yes, but with slightly different meanings:

in das Notizbuch = into the notebook (some notebook already known from context).
ins Notizbuch = contracted form of in das Notizbuch; very common in speech and writing.

Your original sentence uses a possessive:

in mein Notizbuch = into my notebook.

So, three possible versions:

Ich schreibe das Zitat in das Notizbuch. (into that/the notebook)
Ich schreibe das Zitat ins Notizbuch. (same meaning, more natural)
Ich schreibe das Zitat in mein Notizbuch. (into my notebook)

They are all grammatically correct; the possessive mein simply adds ownership.

Why is Notizbuch capitalized?

In German, all nouns are capitalized, regardless of where they appear in the sentence.

Zitat is a noun → Zitat
Notizbuch is a noun → Notizbuch

This is different from English, where only proper nouns and sentence starts are capitalized. In German you must always write nouns with a capital first letter.

Why is it schreibe and not schreiben in this sentence?

Schreiben is the infinitive (to write). In the sentence, the verb is conjugated for the subject ich:

Present tense of schreiben:

ich schreibe
du schreibst
er/sie/es schreibt
wir schreiben
ihr schreibt
sie/Sie schreiben

Since the subject is ich (I), the correct form is ich schreibe.

Can I leave out ich, like in Spanish or Italian, and just say Schreibe das Zitat in mein Notizbuch?

In standard German, you normally do not drop the subject pronoun. You usually have to say ich:

Ich schreibe das Zitat in mein Notizbuch. ✔️

Leaving it out:

Schreibe das Zitat in mein Notizbuch.

This is only correct as a command (imperative), meaning:

(Du) schreibe das Zitat in mein Notizbuch! → Write the quote into my notebook!

So without ich, the meaning changes: it becomes an order to someone else, not a statement about what you are doing.

Can I change the word order, like In mein Notizbuch schreibe ich das Zitat?

Yes, you can, as long as you keep the verb in second position in main clauses.

Possible variants:

Ich schreibe das Zitat in mein Notizbuch.
– Neutral order: subject – verb – object – place.
In mein Notizbuch schreibe ich das Zitat.
– Emphasis on into my notebook; maybe you want to contrast it with somewhere else.
Das Zitat schreibe ich in mein Notizbuch.
– Emphasis on the quote (not something else).

The key rule: in a main clause, the finite verb (schreibe) must stay in second position (counting entire chunks, not single words). You can move other parts around for emphasis or style.

Where would auf go if I use aufschreiben instead of schreiben?

Aufschreiben is a separable verb: auf + schreiben.

In the present tense main clause, auf goes to the end of the sentence:

Ich schreibe das Zitat in mein Notizbuch auf.

Structure:

Ich (subject)
schreibe (finite verb in 2nd position)
das Zitat in mein Notizbuch (objects and adverbial)
auf (separable prefix at the end)

In the infinitive or with a modal verb, it is not split:

Ich will das Zitat in mein Notizbuch aufschreiben.
Ich werde das Zitat in mein Notizbuch aufschreiben.

How would I negate this sentence? Where does nicht go?

The most natural negation is:

Ich schreibe das Zitat nicht in mein Notizbuch.
→ I am not writing the quote into my notebook. (Maybe I am writing it somewhere else.)

Position of nicht:

It usually comes before the part of the sentence that you want to negate.
Here, it negates the prepositional phrase in mein Notizbuch, so it stands right before it.

Other possible focus:

Ich schreibe nicht das Zitat in mein Notizbuch.
→ I am not writing the quote in my notebook (but something else).

Both are grammatically correct, but the meaning/focus is different.

Why is it mein Notizbuch and not meine Notizbuch?

The possessive mein changes form depending on the gender, number, and case of the noun.

Notizbuch is:

grammatical gender: neuter (das Notizbuch)
here: singular, accusative

For neuter singular, nominative and accusative, the form is mein (no extra ending):

Nominative neuter: mein Notizbuch
Accusative neuter: mein Notizbuch

Meine is used for:

feminine singular (nominative/accusative): meine Tasche
plural (all genders, nominative/accusative): meine Notizbücher

So mein Notizbuch is correct here.

What are the plurals of das Zitat and das Notizbuch, and how would the sentence look in plural?

Plurals:

das Zitat → die Zitate (quotations)
das Notizbuch → die Notizbücher (notebooks)

Example sentences:

Plural Zitate, singular notebook:
- Ich schreibe die Zitate in mein Notizbuch. → I write the quotations into my notebook.
Plural notebook, one quote into several notebooks (unusual, but grammatical):
- Ich schreibe das Zitat in meine Notizbücher. → I write the quote into my notebooks.
Both plural:
- Ich schreibe die Zitate in meine Notizbücher. → I write the quotations into my notebooks.

Note how mein becomes meine in the plural: meine Notizbücher.