Learn Hardcore Norwegian: Dagboken er hemmelig, og jeg deler den bare med min beste venninne. - The diary is secret, and I share it only with my best female friend.

It is capitalized here only because it is the first word of the sentence, not because it is a noun (Norwegian does not capitalize all nouns like German).

You could also see dagboka in Bokmål; that is an allowed feminine form. Then the definite would be dagboka, not dagboken. This sentence has chosen the common-gender/masculine pattern (-en).

What gender is dagbok, and how does that affect other words in the sentence?

In Bokmål, dagbok can be treated as either:

common gender (masculine):
- en dagbok – dagboken
feminine:
- ei dagbok – dagboka

In the sentence, dagboken is used, so the writer is treating dagbok as common gender/masculine.

This affects the pronoun later in the sentence:

jeg deler den bare med …
Here den refers back to dagboken.
den is used for common-gender nouns (and masculine), while det is used for neuter nouns.

If someone used the feminine form dagboka, many speakers would still say den, but in very feminine-heavy speech you might hear ho (she) instead of den; however, in standard Bokmål writing, den is normal.

Why is the adjective hemmelig in that form? Why not hemmelige or hemmeligt?

hemmelig means “secret”. In this sentence, it is used as a predicative adjective after the verb er (“is”):

Dagboken er hemmelig. = “The diary is secret.”

For adjectives in this position (after er, blir, etc.) and referring to a singular subject, you normally use the base form:

Boka er ny. (The book is new.)
Filmen er interessant. (The movie is interesting.)
Dagboken er hemmelig. (The diary is secret.)

Forms with -e and -t are used in other situations:

hemmelig
- neuter singular noun: et hemmelig brev (a secret letter)
hemmelig
- definite singular / plural noun: den hemmelige planen, hemmelige brev

Here we just have “X er [adjective]” with a singular subject, so hemmelig is the correct form.

Why is there a comma before og in “… hemmelig, og jeg deler …”? Do you always put a comma before og?

No, you do not always put a comma before og.

In this sentence, og connects two main (independent) clauses:

Dagboken er hemmelig
jeg deler den bare med min beste venninne

When og links two full clauses with their own subject and verb, a comma before og is usually correct (and in many style guides, preferred):

Jeg er trøtt, og jeg skal legge meg.
“I am tired, and I’m going to bed.”

But you do not put a comma before og when it links simple items in a list or two verbs sharing one subject:

Jeg kjøper melk og brød. (no comma)
Han sitter og leser. (no comma)

So in this sentence, the comma is there because we have two complete clauses joined by og.

Why do we say jeg deler den instead of repeating dagboken, and is that normal in Norwegian?

Using the pronoun den instead of repeating dagboken is very natural in Norwegian and avoids repetition:

Dagboken er hemmelig, og jeg deler den bare med min beste venninne.
“The diary is secret, and I only share it with my best (female) friend.”

You could grammatically say:

Dagboken er hemmelig, og jeg deler dagboken bare med min beste venninne.

…but that sounds heavy and repetitive. Native speakers almost always replace the second dagboken with a pronoun (den), just like English uses it.

Why is it den and not det in jeg deler den bare med …?

Norwegian object pronouns den / det must match the grammatical gender of the noun they refer to:

den → common gender (masculine/feminine in Bokmål)
det → neuter

Since dagboken is common gender (from en dagbok), the correct pronoun is:

jeg deler den = “I share it”

If the noun were neuter, you would use det:

brevet er hemmelig, og jeg deler det bare med …
(“the letter is secret, and I only share it with …”, where brev is neuter: et brev – brevet – det)

Why is bare placed after deler in jeg deler den bare med …? Could it go somewhere else?

The default rule in Norwegian main clauses is:

Subject – Verb – (Object) – Mid-position adverbs (like bare) – Rest

In this sentence:

Subject: jeg
Verb: deler
Object: den
Adverb: bare
Prepositional phrase: med min beste venninne

So the neutral word order is:

Jeg deler den bare med min beste venninne.
= “I only share it with my best (female) friend.”

Other placements are possible but can change the focus or sound less natural:

Jeg bare deler den med min beste venninne.
Can sound like you’re defending yourself: “I only share it with my best friend (that’s all I do).”
Jeg deler bare den med min beste venninne.
Emphasizes den: “I share only that (one) with my best friend (not others).”

The sentence you have is the normal, neutral way to say “I only share it with my best (female) friend.”

Why is the verb deler and not dele?

dele is the infinitive (“to share”). Norwegian verbs in the present tense (for all persons) usually end in -r:

å dele (to share) → deler (share / shares)
å spise (to eat) → spiser (eat / eats)
å bo (to live) → bor (live / lives)

Norwegian does not change the verb form for different persons:

jeg deler (I share)
du deler (you share)
han/hun deler (he/she shares)
vi deler (we share)
de deler (they share)

So jeg deler den … = “I share it …” in the present tense.

Why is the preposition med used in deler den … med min beste venninne? Why not til?

In Norwegian, the usual pattern is:

å dele noe med noen = “to share something with someone”

So you say:

Jeg deler hemmeligheter med deg.
Vi deler leilighet med en venn.

The preposition til means “to” and is used for giving or sending something to someone, not for sharing:

Jeg gir boka til henne. (I give the book to her.)
Han sendte en e-post til sjefen. (He sent an email to the boss.)

If you said dele noe til noen, it would sound wrong or at best very odd. The correct collocation is dele … med ….

What’s the difference between venn and venninne, and why is venninne used here?

venn = friend (traditionally gender-neutral / masculine)
venninne = female friend

venninne is used when you specifically want to say that the friend is female. So:

min beste venn = my best friend (gender-neutral, can be male or female)
min beste venninne = my best female friend

In modern usage, many people use venn for everyone and avoid venninne, especially in more gender-neutral language. But venninne is still very common and completely natural if the speaker wants to highlight that the friend is a woman/girl.

So in this sentence, min beste venninne explicitly means “my best female friend.”

Why is it min beste venninne and not min venninne beste or something like that? How does word order work with min and beste?

The normal word order with a possessive and an adjective is:

possessive – adjective – noun

So:

min beste venninne = my best (female) friend
min gode venn = my good friend
hans gamle bil = his old car

You cannot put beste after the noun the way English sometimes can (“friend of mine, who is best”), so forms like min venninne beste are incorrect.

Norwegian does allow the possessive after the noun in many cases:

venninna mi = my (female) friend
den beste venninna mi = my best (female) friend

But if the possessive comes before the noun, the pattern is:

min beste venninne
min gode venn
mitt lille barn

So min beste venninne is exactly the normal, natural order.

What does beste mean here? Is it a special form?

beste is the superlative form of the adjective god (“good”) when used in front of a noun:

god = good
bedre = better (comparative)
best / beste = best (superlative)

In front of a noun, the superlative usually appears as beste:

min beste venninne = my best (female) friend
den beste filmen = the best movie
et av de beste minnene = one of the best memories

So min beste venninne literally means “my best (female) friend,” not just “my good (female) friend.”