Learn Hardcore Norwegian: Da jeg var tenåring, pleide jeg å skrive dikt i stedet for dagbok. - When I was a teenager, I used to write poems instead of a diary.

da is normally used for:
- a single event in the past
- or a finished time period in the past
når is used for:
- general, repeated events (whenever)
- present and future time
- questions about time

Being a teenager is a specific (finished) period in the past, so da is the natural choice:

Da jeg var tenåring, pleide jeg å skrive dikt ...
= When I was a teenager, I used to write poems ...

Using Når jeg var tenåring would sound wrong to a native speaker in this context.

Why is there no article before tenåring? Why not en tenåring?

After the verb å være (å være = to be), Norwegian usually drops the article when you describe:

a profession: Jeg er lærer. (I am a teacher.)
a role/status: Hun er mor. (She is a mother.)
a life stage: Han er student. (He is a student.)

So:

Jeg var tenåring.
literally I was teenager., but it means I was a teenager.

If you say Jeg var en tenåring, it is grammatically possible, but it sounds more like:

I was one (particular) teenager (for example, in a group of teenagers)

In this sentence, we are just talking about the life stage in general, so no article is used: tenåring, not en tenåring.

Why is it pleide jeg å skrive and not jeg pleide å skrive?

This is about word order in Norwegian main clauses.

Norwegian has V2 word order: the finite verb (here: pleide) must be in the second position in a main clause.

Your sentence has two parts:

Subordinate clause: Da jeg var tenåring,
Main clause: pleide jeg å skrive dikt i stedet for dagbok.

When a subordinate clause comes first, the next clause must still keep the verb in second position:

Da jeg var tenåring, pleide jeg å skrive dikt ...
(1st element = the whole time clause Da jeg var tenåring,
2nd element = verb pleide,
3rd element = subject jeg)

If you said Da jeg var tenåring, jeg pleide å skrive dikt, the verb would not be in second position in the main clause, which is incorrect word order in standard Norwegian.

What does pleide å skrive mean exactly? How is it different from just skrev?

Both can talk about the past:

skrev = wrote (simple past)
pleide å skrive = used to write (a past habit)

pleide å + infinitive is used for actions that were:

regular / habitual in the past
but are not done anymore (or we don’t focus on the present)

So:

Da jeg var tenåring, skrev jeg dikt.
= I wrote poems when I was a teenager. (a statement about the past; might be a habit, but it’s not highlighted)
Da jeg var tenåring, pleide jeg å skrive dikt.
= When I was a teenager, I used to write poems. (clearly habitual or typical behaviour)

In many contexts, you can use skrev instead of pleide å skrive and still be understood. pleide å just makes it extra clear that it was a repeated habit.

Why do we need å in pleide jeg å skrive?

In Norwegian, many verbs are followed by another verb in the infinitive form. The infinitive is usually marked with å, similar to English to:

liker å lese = like to read
begynner å løpe = start to run
prøver å sove = try to sleep

pleide belongs to this group:

pleide å skrive = used to write

So the pattern is: pleide + å + infinitive.
Without å (pleide jeg skrive) it sounds wrong in standard Norwegian.

What is the difference between dikt and poesi? And why is it dikt here?

Both relate to poetry, but they’re used differently:

dikt
- can mean a poem (countable):
  - et dikt = a poem
  - flere dikt = several poems
- is also used as a mass noun for poetry in general in many contexts
poesi
- is more abstract/literary
- means poetry as an art form or genre, not individual poems

In everyday language, Norwegians often say:

skrive dikt (to write poems / poetry)

So å skrive dikt is the most natural choice in this personal, everyday sentence.
å skrive poesi would sound a bit more high‑flown or artistic.

Why is it i stedet for dagbok and not something like istedenfor dagbok?

The standard form in Bokmål is:

i stedet for = instead of

It is written as three separate words.

You will see informal spellings like istedenfor (one word) in some texts, but in careful / standard writing, i stedet for is preferred.

So:

i stedet for dagbok = instead of (a) diary
The structure is: i stedet for + noun / pronoun / phrase

Why is there no article before dagbok? Could we say i stedet for en dagbok?

Both are possible, but there is a nuance.

i stedet for dagbok
- talking about the activity in general:
- instead of keeping a diary / instead of a diary [as a practice]
i stedet for en dagbok
- more concrete: one specific diary as an object
- instead of a (particular) diary

In this sentence, we are contrasting ways of expressing yourself (writing poems vs keeping a diary), so the general, article‑less dagbok is more natural:

... å skrive dikt i stedet for dagbok.
= ... to write poems instead of (keeping) a diary.

Why is there a comma after Da jeg var tenåring?

Norwegian punctuation rules require a comma between:

a subordinate clause and
a main clause

Here:

Da jeg var tenåring = subordinate clause (a time clause)
pleide jeg å skrive dikt i stedet for dagbok = main clause

So we write:

Da jeg var tenåring, pleide jeg å skrive dikt ...

If you remove the comma, it will often still be understood, but it is not correct according to standard Norwegian comma rules.

Could I say Som tenåring pleide jeg å skrive dikt instead? Is that more natural?

Yes, that is also natural, and many speakers might prefer it in casual speech:

Som tenåring pleide jeg å skrive dikt i stedet for dagbok.
= As a teenager, I used to write poems instead of a diary.

Differences:

Da jeg var tenåring, ...
- explicit full clause with subject and verb (jeg var)
- a bit more formal or book‑like
Som tenåring, ...
- shorter, adjectival phrase (as a teenager)
- very common and natural in spoken and written Norwegian

Both are fully correct; the meaning is effectively the same in this context.

Is tenåring the only way to say teenager, and is it gender‑neutral?

tenåring is the standard, gender‑neutral Norwegian word for teenager.

en tenåring = a teenager
tenåringen = the teenager
flere tenåringer = several teenagers
tenåringene = the teenagers

It does not mark gender; you need extra words if you want that:

en tenåringsgutt = a teenage boy
en tenåringsjente = a teenage girl

In Da jeg var tenåring, it is fully neutral: it just states that the speaker was in their teenage years, regardless of gender.