Learn Hardcore Norwegian: Jeg skriver i dagboken min hver kveld før jeg legger meg. - I write in my diary every evening before I go to bed.

After the noun (very common, often neutral or slightly informal):
- dagboken min = my diary
- literally: the-diary my
Before the noun (more emphatic, or in some contexts more formal/written):
- min dagbok = my diary (with a bit more emphasis on my)

In your sentence, dagboken min is the most natural everyday wording.
Min dagbok is not wrong, but it would sound more marked, like stressing whose diary it is, or more formal/literary depending on context.

Why is it dagboken (definite form) and not just dagbok?

Norwegian uses the definite form of many nouns in cases where English uses the indefinite or leaves it bare.

dagbok = a diary / diary (in general)
dagboken = the diary

When you say you write in your diary, you’re normally referring to a specific, known diary. In Norwegian, that pushes the noun into definite form:

i dagboken min = in my diary
literally: in the diary of-mine

Saying i dagbok would sound like in a diary in a very generic sense, and is not idiomatic here.

Why do we say i dagboken min and not på dagboken min?

The preposition i means in / inside, and på usually means on / on top of.

You write in a diary (inside its pages), so Norwegian uses:

skrive i dagboken min = write in my diary

You would use på with things you write on:

skrive på tavla = write on the board
skrive på et ark (colloquial) = write on a sheet (of paper)

So i dagboken min matches the idea of being inside the diary, page by page.

Why is it skriver and not something like er å skrive or a special form for “am writing”?

Norwegian does not have a separate present continuous form like English (I am writing). The simple present skriver can mean both:

Jeg skriver i dagboken min.
= I write in my diary. (habitual) = I am writing in my diary. (right now), depending on context.

The sentence Jeg skriver i dagboken min hver kveld is clearly about a habit because of hver kveld (every evening), so skriver is understood as write (regularly).

What does hver kveld literally mean, and why not just kveld?

hver = every / each
kveld = evening

So hver kveld = every evening.

If you used just kveld (evening), it would not express repetition. Hver is needed to show that this happens regularly, every day:

Jeg skriver i dagboken min hver kveld.
= I write in my diary every evening.

What’s the difference between hver kveld and om kvelden?

Both can describe something that usually happens in the evening, but the nuance is different:

hver kveld = every evening, clearly day-by-day repetition.
om kvelden = in the evenings / in the evening (generally), more like a general time of day.

Examples:

Jeg skriver i dagboken min hver kveld.
= I write every single evening, regularly.
Jeg liker å lese om kvelden.
= I like to read in the evenings (as a general habit, not strongly emphasizing every single evening).

Why do we say før jeg legger meg and not just før jeg legger?

The verb here is å legge seg = to go to bed / to lie down (to sleep), literally lay oneself down.
This is a reflexive verb, so it needs a reflexive pronoun:

jeg legger meg = I go to bed / I lay myself down
du legger deg = you go to bed
han legger seg = he goes to bed

If you say jeg legger without the reflexive, it means I put/lay (something), and you must say what you lay down:

Jeg legger boka på bordet. = I put the book on the table.

So in før jeg legger meg, meg is required for the meaning go to bed.

Why is it jeg legger meg and not jeg meg legger or some other word order?

In a normal main clause, Norwegian word order is:

Subject – Verb – (Object / other elements)

So:

Jeg (subject) legger (verb) meg (reflexive object)

When you add før (before), you are introducing a subordinate clause, but in this particular type of clause in spoken/written Bokmål, the order after før is still:

før jeg legger meg
= før
- subject (jeg) + verb (legger) + object (meg)

You do not move the reflexive meg in front of the verb; it stays after the verb.

Why is it meg and not meg selv in jeg legger meg?

Meg selv means myself with extra emphasis, similar to English:

Jeg så meg selv i speilet. = I saw myself in the mirror.

For reflexive verbs like å legge seg, the simple reflexive pronoun (meg, deg, seg, etc.) is the standard form:

jeg legger meg = I go to bed
jeg vasker meg = I wash (myself)
jeg setter meg = I sit down

Using meg selv here would sound over-emphatic or unnatural:
jeg legger meg selv is not how you say I go to bed.

Could you also say Jeg skriver i dagboken min hver natt instead of hver kveld?

You could, but it changes the meaning slightly:

hver kveld = every evening (before or around bedtime, but still evening)
hver natt = every night (usually during the night, after you’ve gone to bed or when it’s dark)

Your original sentence suggests a routine in the evening before sleeping.
If you said hver natt, it would sound more like you write late at night, possibly during the night itself, which is a different time frame.

Is there a difference between i dagboken min and i min dagbok, or are they the same?

Both are grammatically correct and mean in my diary, but:

i dagboken min
– More common, neutral, natural everyday Norwegian.
– Definite noun (dagboken) + possessive after.
i min dagbok
– Sounds more emphatic or stylistically marked (e.g. literary, contrastive).
– Possessive before the noun, usually with dagbok in indefinite form, but still understood as my diary.

In most normal speech and writing about habits, i dagboken min is the default choice.