Jeg fant et gammelt oppskrifthefte med sunne oppskrifter til lunsj.

Because oppskrifthefte is a neuter noun.

• hefte is neuter: et hefte (a booklet).
• In a compound like oppskrifthefte, the gender is determined by the last part (hefte), so the whole word is neuter: et oppskrifthefte.

Adjectives must agree with the gender and number of the noun:

• Masculine/feminine singular: en gammel stol (a old chair)
• Neuter singular: et gammelt hefte (an old booklet)
• Plural (all genders): gamle hefter (old booklets)

So you need et gammelt oppskrifthefte, not en gammel oppskrifthefte.

Oppskrifthefte is a compound noun:

• oppskrift = recipe
• hefte = booklet / pamphlet

Put together: oppskrifthefte ≈ recipe booklet.

In Norwegian, compounds are very common: two (or more) nouns are joined into one word to make a more specific concept, and the last part determines:

• the gender,
• the plural endings,
• the article.

So:

• Singular indefinite: et oppskrifthefte
• Singular definite: oppskriftheftet
• Plural indefinite: oppskrifthefter
• Plural definite: oppskriftheftene

You will often see oppskriftshefte, and that is probably the more common and more natural form.

• oppskrift
  • linking s
    • hefte → oppskriftshefte

Norwegian frequently uses a linking s inside compound nouns. Both oppskrifthefte and oppskriftshefte are understandable; oppskriftshefte just sounds more idiomatic to many native speakers.

The grammar (gender, adjective agreement) is the same for both, because the last element is still hefte (neuter).

Because oppskrifter is plural, and the adjective must agree with that.

• oppskrift (singular): en oppskrift
• oppskrifter (plural): (no article) oppskrifter, recipes

Adjective sunn (healthy):

• Singular indefinite masc/fem: sunn (en sunn oppskrift)
• Singular indefinite neuter: sunt (et sunt måltid)
• Plural indefinite (all genders): sunne (sunne oppskrifter)

So:

• sunn oppskrifter is wrong (adjective is singular, noun is plural).
• sunne oppskrift is wrong (adjective is plural, noun is singular).
• sunne oppskrifter is correct: both are plural.

In this context, til lunsj means roughly for lunch or for lunchtime meals.

The phrase sunne oppskrifter til lunsj can be understood as:

• healthy recipes for lunch,
• healthy recipes for making lunch.

Norwegian often uses til with meals:

• til frokost – for breakfast
• til lunsj – for lunch
• til middag – for dinner
• til kveldsmat – for supper / evening meal

It usually expresses that something is intended to be eaten as that meal.

In normal reading, til lunsj belongs to sunne oppskrifter:

• sunne oppskrifter til lunsj = healthy recipes for lunch.

Norwegian tends to attach prepositional phrases like til lunsj to the closest suitable noun that makes sense. Here, it clearly fits oppskrifter.

If you wanted to say I found it at lunchtime, you would normally phrase it differently, for example:

• Jeg fant det i lunsjpausen. – I found it during the lunch break.
• Jeg fant det i lunsjen. – I found it at lunch / during lunch.

So in the original sentence there’s not really a natural ambiguity: til lunsj describes the type of recipes.

Yes, you can, and it’s quite natural.

• sunne oppskrifter til lunsj = healthy recipes for lunch
• sunne lunsjoppskrifter = healthy lunch-recipes

lunsjoppskrifter is another compound noun:

• lunsj (lunch) + oppskrifter (recipes) → lunsjoppskrifter

The meaning is nearly the same. Sunne lunsjoppskrifter is a bit more compact and sounds like a specific category of recipes (the “lunch recipes” section), while sunne oppskrifter til lunsj explicitly spells out recipes for lunch.

Fant is the preterite (simple past) of å finne (to find):

• infinitive: å finne
• present: finner
• preterite (simple past): fant
• past participle: funnet

So Jeg fant = I found.

Norwegian uses the simple past much like English does:

• Jeg fant et gammelt oppskrifthefte … – I found an old recipe booklet …
• I går fant jeg … – Yesterday I found …

You could also say:

• Jeg har funnet et gammelt oppskrifthefte … – I have found an old recipe booklet …

Both are possible; the choice depends on context and nuance, much like I found vs I have found in English.

You can say it, but the meaning becomes unclear or different.

Original:

• … et gammelt oppskrifthefte med sunne oppskrifter til lunsj.
  • Clear: a recipe booklet with healthy recipes for lunch.

Your variant:

• … et gammelt oppskrifthefte til lunsj med sunne oppskrifter.

Now til lunsj is closer to oppskrifthefte, so it can sound like:

• a recipe booklet for lunch (maybe a booklet you use at lunchtime?),
• instead of recipes for lunch.

In Norwegian, it’s best to keep:

• adjectives directly before their noun, and
• prepositional phrases as close as possible to the word they logically belong to.

So the original word order is clearer and more natural.

Med in this sentence means roughly with or containing:

• et oppskrifthefte med sunne oppskrifter
  = a recipe booklet with healthy recipes / containing healthy recipes.

You could also express this with som har:

• et gammelt oppskrifthefte som har sunne oppskrifter til lunsj.
  = an old recipe booklet that has healthy recipes for lunch.

Both are grammatically fine, but:

• med + noun is shorter and more natural for describing what something contains.
• som har sounds a bit more like a relative clause, and is slightly heavier stylistically.

Gammel (old) changes form depending on gender, number, and definiteness:

Indefinite singular:

• Masculine/feminine: en gammel stol – an old chair
• Neuter: et gammelt hus – an old house

Indefinite plural:

• All genders: gamle stoler/hus – old chairs/houses

Definite forms:

• Masculine/feminine: den gamle stolen – the old chair
• Neuter: det gamle huset – the old house
• Plural: de gamle stolene – the old chairs

In your sentence:

• et gammelt oppskrifthefte – neuter singular → gammelt.

Sunn (healthy) follows the regular pattern for many adjectives:

Indefinite singular:

• Masculine/feminine: en sunn frokost – a healthy breakfast
• Neuter: et sunt måltid – a healthy meal

Indefinite plural:

• All genders: sunne måltider – healthy meals

Definite forms:

• Masculine/feminine: den sunne frokosten
• Neuter: det sunne måltidet
• Plural: de sunne måltidene

In your sentence, oppskrifter is plural, so you must use:

• sunne oppskrifter – healthy recipes.

Here are the main forms in Bokmål:

oppskrift (recipe) – masculine (often treated as common gender):

• Indefinite singular: en oppskrift
• Definite singular: oppskriften
• Indefinite plural: oppskrifter
• Definite plural: oppskriftene

hefte (booklet / pamphlet) – neuter:

• Indefinite singular: et hefte
• Definite singular: heftet
• Indefinite plural: hefter
• Definite plural: heftene

For the compound oppskrift(s)hefte, you follow the pattern of hefte, since the last element controls the forms:

• et oppskrift(s)hefte, oppskrift(s)heftet, oppskrift(s)hefter, oppskrift(s)heftene.