Learn Hardcore German: Am Ende des Tages schreibe ich alle Regeln in mein Notizbuch. - At the end of the day I write all the rules in my notebook.

In German you need to specify “the end of the day” using “Ende” (which is neuter) plus a genitive attribute. So you say “das Ende des Tages”. And since “am” is the contraction of “an dem” (at the), you get “Am Ende des Tages”. Simply saying “Am Ende Tag” would leave out both the article for “Tag” and the necessary genitive link.

Why is des Tages in the genitive case instead of dative or accusative?

The phrase “Ende des Tages” literally means “end of the day.” In German, to express “of” in this sense you use the genitive. Even though “am” (an + dem) puts “Ende” itself into the dative, the relationship “Ende → Tages” is still shown by the genitive.

Why does schreibe come before ich in this sentence?

German is a verb-second language. When you start a sentence with a time expression like “Am Ende des Tages,” that whole phrase occupies position 1. The finite verb must then occupy position 2, forcing the subject “ich” into position 3. Hence:
Position 1: Am Ende des Tages
Position 2: schreibe
Position 3: ich

Why is it alle Regeln and not allen Regeln or die Regeln?

Here “alle” is a quantifier meaning “all” and takes the accusative plural because “Regeln” is the direct object of “schreibe.” In the accusative plural, “alle” does not change its ending (it stays alle), so you get “alle Regeln.” If you wanted to say “the rules,” you’d use “die Regeln.”

Why do we use in mein Notizbuch with the accusative case?

The preposition “in” in German can take either dative or accusative. You use the accusative when there is movement or direction into something. Writing into a notebook implies direction (you are putting your rules onto the pages), so you need the accusative:
in + das Notizbuch → in mein Notizbuch

Could I reorder the sentence as Ich schreibe alle Regeln am Ende des Tages in mein Notizbuch? Would it change the meaning?

Yes, you can. German allows you to move time, manner, place phrases in the sentence as long as the verb stays in second position. Both orders mean the same thing. Putting “Ich” first sounds more neutral, while starting with “Am Ende des Tages” emphasizes when you do it.

Why is Notizbuch one word, and why is its possessive mein?

German loves compound nouns: two or more words join to make a new noun. “Notiz” (note) + “Buch” (book) = Notizbuch (notebook). Compounds are always capitalized as one word.
Since Notizbuch is neuter (das Notizbuch), the possessive adjective “mein” also stays in the neuter form:
“Nominative/Accusative neuter” = mein Notizbuch.