Learn Hardcore Norwegian: På festen i helgen fikk fetteren min mye oppmerksomhet for musikken sin. - At the party this weekend my male cousin got a lot of attention for his music.

Using i festen would sound wrong here; i is used more for being inside something physical (a room, a box, a car, a building), or for longer periods (i sommer, i 2020). A party is treated as an event, not a container, so you use på.

What exactly does i helgen mean? Is it past or future?

i helgen literally means in/over the weekend. It can refer to:

the coming weekend
the previous weekend

Which one it means depends on context and the verb tense:

Jeg skal jobbe i helgen. – I’m going to work this weekend. (future from context)
Jeg jobbet mye i helgen. – I worked a lot this weekend / last weekend. (past from context)

In your sentence the verb is past (fikk), so på festen i helgen will almost always be understood as at the party this past weekend / at the party over the weekend.

Could I say i helga instead of i helgen?

Yes. Both are correct Bokmål:

i helgen – more common in Eastern/Bokmål-standard speech
i helga – very common in many dialects and in Nynorsk; also accepted in Bokmål

They mean the same thing; the difference is mainly regional/style.

Why is it fetteren min and not min fetter?

Both are grammatically correct, but they differ in style:

fetteren min – possessive after the noun; this is the most neutral, everyday word order in Norwegian.
min fetter – possessive before the noun; this can sound a bit more emphatic, formal, or written.

In ordinary conversation you’ll most often hear:

fetteren min, broren min, vennene mine, boka mi, etc.

Why is it fetteren (definite form) and not just fetter?

Norwegian normally uses the definite form when you talk about a specific, known person:

fetteren min – my (specific) cousin
en fetter – a cousin (not specified which one)

Because you’re referring to my cousin (a particular person), not just some cousin, you use the definite: fetteren + min.

Forming the definite for this masculine noun:

en fetter – a cousin
fetteren – the cousin

Why is the subject phrase so late: På festen i helgen fikk fetteren min ...? Can I say Fetteren min fikk ... på festen i helgen?

Yes, you can say:

Fetteren min fikk mye oppmerksomhet for musikken sin på festen i helgen.

Both orders are correct. The difference is emphasis and information structure:

På festen i helgen fikk fetteren min ...
Starts by setting the scene in time and place, then introduces what happened there.
Fetteren min fikk ... på festen i helgen.
Starts with the person and adds where/when at the end.

Norwegian main clauses like to have the verb in second position (V2 rule). In your original sentence:

På festen i helgen – first element
fikk – verb in second position
fetteren min – comes after the verb

So the word order is actually following a strong Norwegian rule.

What does fikk correspond to in English, and why is it used here?

fikk is the past tense of få (to get, receive).

få – to get, receive
fikk – got, received

The phrase å få oppmerksomhet literally means to get/receive attention. So:

fikk mye oppmerksomhet – got a lot of attention / received a lot of attention

Even though English often says got a lot of attention or had a lot of attention, Norwegian standardly uses få here.

Why is it mye oppmerksomhet and not mange oppmerksomheter?

oppmerksomhet (attention) is used as an uncountable noun in Norwegian, like water or music often are:

mye oppmerksomhet – a lot of attention
lite oppmerksomhet – little attention

For uncountable nouns, you use mye (a lot of) / lite (a little).
mange (many) is used with countable plural nouns:

mange venner – many friends
mange sanger – many songs

So mange oppmerksomheter is not idiomatic; you almost never pluralize oppmerksomhet in everyday language.

Why is it for musikken sin? Does for here mean for, about, or because of?

In this context, for means roughly “because of / for” in the sense of receiving praise or attention for something:

få ros for noe – get praise for something
få en pris for noe – get a prize for something
få mye oppmerksomhet for musikken sin – get a lot of attention for his music

So for musikken sin means for his music in the sense because of his music; on the basis of his music.

What is the difference between musikk and musikken here?

musikk – music (in general, indefinite)
musikken – the music (definite, specific)

In for musikken sin, we are talking about his particular music, the music he played/made, not music in general. That’s why the definite form musikken is used.

If you said for musikk, it would sound incomplete or too general, like “for music” as an abstract thing, not his music at that event.

Why is it musikken sin and not musikken hans? What does sin do?

Norwegian has reflexive possessive pronouns (sin, si, sitt, sine) which indicate that something belongs to the subject of the clause.

In your sentence:

Subject: fetteren min
Possession: musikken
Owner = subject → use a reflexive possessive: sin

So:

for musikken sin – for his own music (the cousin’s music)

If you used musikken hans, it usually means the music that belongs to some other male person, not the subject. Compare:

Fetteren min fikk mye oppmerksomhet for musikken sin.
→ My cousin got a lot of attention for his own music.
Fetteren min fikk mye oppmerksomhet for musikken hans.
→ My cousin got a lot of attention for his music – i.e. some other guy’s music (not my cousin’s).

So sin ties the ownership back to the subject of the same clause.

How do the different forms of sin work (sin, si, sitt, sine)?

They agree with the gender and number of the noun owned, not with the person who owns it:

sin – with masculine and feminine singular nouns
- boka si (fem.), bilen sin (masc.)
si – (used with some feminine nouns in certain styles/dialects; in standard Bokmål many people just use sin as the common form)
sitt – with neuter singular nouns
- huset sitt – his/her own house
sine – with plural nouns
- vennene sine – his/her own friends

In your sentence:

musikk is masculine in form when made definite (musikken)
so you use sin → musikken sin

Is oppmerksomhet countable in any context, or always uncountable?

In normal, everyday usage oppmerksomhet is treated as uncountable, so you say:

få mye oppmerksomhet – get a lot of attention
vise noen oppmerksomhet – show someone attention

A rare, plural use (oppmerksomheter) might appear in very formal or poetic language to mean gestures of attention or attentions, but this is not common. For a learner, it’s safest to treat oppmerksomhet as uncountable and use mye/lite, not mange/få.