Learn Hardcore Swedish: På kvällen skriver hon några rader i sin dagbok om hur dagen har varit. - In the evening she writes a few lines in her diary about how the day has been.

The finite verb (here skriver) must be the second element in a main clause.
If you start the sentence with something other than the subject (here På kvällen), that counts as the first element, so the verb must come next, and the subject comes after the verb.

So:

Neutral word order (subject first):
- Hon skriver några rader i sin dagbok. – She writes a few lines in her diary.
With a time adverbial first:
- På kvällen skriver hon några rader i sin dagbok.

This is normal Swedish word order and not emphasis in the same way it would be in English.

Why is it kvällen (definite form) and not en kväll or just kväll?

When you talk about a repeated or typical time of day in a general way, Swedish often uses the definite form:

På kvällen – in the evening (in general, in the evenings)
På morgonen – in the morning
På sommaren – in (the) summer (as a recurring season)

You could say på en kväll but that would mean something like “in one (specific) evening”, focusing on a single evening rather than a habitual time.

So På kvällen skriver hon… means “In the evening(s), she writes…”, describing a regular action or a typical time, which fits the definite form kvällen.

What does några mean, and how is några rader different from just rader?

Några means “a few / some (countable)”. It’s the plural form of någon (“some/any” for one item).

rad – (one) line
rader – lines
några rader – a few lines / some lines

If you said just rader, you’d usually need something to specify how many or which ones (e.g. flera rader, alla rader).

Några rader is a natural way to say she only writes a little, not a whole long entry.

Why is it några rader and not några rad?

Because rad (“line”) is a countable noun and must take plural when it refers to more than one:

1 line: en rad
2 lines: två rader
a few lines: några rader

Några always goes with a plural noun. So några rad would be grammatically wrong.

Why do we say i sin dagbok (“in her diary”) and not på sin dagbok?

The preposition depends on how Swedish conceptualizes the noun:

For books, diaries, notebooks, Swedish normally uses i (in), because you write in them:
- i boken – in the book
- i anteckningsboken – in the notebook
- i sin dagbok – in her diary

På is used with surfaces or things you write on:

på tavlan – on the board
på ett papper – on a sheet of paper

A diary is thought of as something you write inside, so i is the natural preposition: i sin dagbok.

What is the difference between sin dagbok and hennes dagbok? Why is sin used here?

Swedish has reflexive possessive pronouns (sin, sitt, sina) that refer back to the subject of the clause:

sin – “his/her/its/their own” with en-words (like dagbok)
sitt – with ett-words
sina – with plural nouns

In this sentence, the subject is hon (“she”). Sin dagbok therefore means her own diary (the diary belonging to the subject “she”).

Hon skriver i sin dagbok. – She writes in her own diary.
Hon skriver i hennes dagbok. – She writes in some other woman’s diary.

So sin is used to show that the diary belongs to the subject hon. If the diary belonged to another woman, you would use hennes.

How do sin / sitt / sina change with different nouns?

They agree with the grammatical form of the noun possessed, not with the owner:

sin – with en-words (common gender, singular):
- sin bok – his/her own book
- sin dagbok – his/her own diary
sitt – with ett-words (neuter, singular):
- sitt hus – his/her own house
- sitt brev – his/her own letter
sina – with plural nouns (both genders):
- sina böcker – his/her own books
- sina dagböcker – his/her own diaries

So we say sin dagbok because dagbok is an en-word: en dagbok.

What is the function of om in om hur dagen har varit? Is it like “about”?

Yes. Here om means “about”:

skriva om något – to write about something
berätta om något – to tell about something

So:

…skriver hon några rader i sin dagbok om hur dagen har varit.
= she writes a few lines in her diary about how the day has been.

You could also say simply:

Hon skriver om sin dag. – She writes about her day.

Why is it om hur dagen har varit (“about how the day has been”) and not just om dagen? What does hur add?

Hur means “how”, and here it introduces a description of the day, not just the topic:

om dagen – about the day (just the day in general; very broad)
om hur dagen har varit – about how the day has been (what it was like, how it went, how she experienced it)

So hur turns it into “the manner or quality of the day” rather than just “the day” as a subject.

The whole phrase hur dagen har varit is a subordinate clause (“how the day has been”) that functions as the object of om.

Why is the verb phrase har varit used instead of just var for “was / has been”?

Har varit is the present perfect:

har (present of “to have”) + varit (supine of vara = “to be”).

In Swedish, the present perfect is often used to describe something that:

Happened earlier (in the past), and
Is still relevant to “now” or is connected to the present situation.

At the time she writes in the evening, the day is just ending, so its events are fresh and relevant to the present moment. That’s why har varit feels natural:

Om hur dagen har varit. – About how the day has been (up until now).

If you said om hur dagen var, it would sound a bit more detached or like a completed block of time, with less focus on its connection to the present. It isn’t wrong, but har varit is more idiomatic here.

Can På kvällen go at the end of the sentence? How does that change the word order?

Yes, you can move the time expression, and the word order will change:

På kvällen skriver hon några rader i sin dagbok om hur dagen har varit.
– Time adverbial first → verb in second position (skriver), subject after verb (hon).
Hon skriver några rader i sin dagbok om hur dagen har varit på kvällen.
– Subject first (Hon), so Hon is first element and skriver is still in second position. På kvällen now comes later.

Both are grammatically fine. Putting På kvällen first makes the time frame a bit more prominent, like setting the scene: “In the evening, she writes…”.

Is the tense of skriver present or something else? How does it relate to har varit?

Skriver is present tense (“writes” / “is writing”). So the structure is:

På kvällen skriver hon… – In the evening, she writes… (present – describing a regular or current habit)
…om hur dagen har varit. – …about how the day has been. (present perfect – the day up to that point)

This combination is natural: a present habitual action (skriver) referring back to something that just happened earlier that same day (har varit).

You could also adjust both to the past if you wanted a narrative in past tense:

På kvällen skrev hon några rader i sin dagbok om hur dagen hade varit.
– “In the evening, she wrote a few lines in her diary about how the day had been.” (past + past perfect)