Learn Hardcore Icelandic: Vinátta okkar hefur góð áhrif á líf okkar. - Our friendship has a good effect on our life.

It is much less common.
It usually gives extra emphasis or is used in some stylistic/poetic ways.
With okkar specifically, okkar vinátta would feel marked or emphatic, something like “our friendship (as opposed to others’ friendship)”.

So the neutral, everyday choice is vinátta okkar.

Why do we use okkar twice: vinátta okkar and líf okkar? Is that necessary?

Yes, repeating okkar is normal and natural here.

vinátta okkar = our friendship
líf okkar = our life / our lives

If you omit the second okkar and just say:

Vinátta okkar hefur góð áhrif á líf.

this would sound like “Our friendship has a good influence on life (in general),” not clearly our life.

So if you specifically mean our lives, you normally repeat the possessive:

…á líf okkar.

Why do we use hefur here? Could we say something like er með instead?

hefur is the 3rd person singular of the verb hafa (to have).

Vinátta okkar hefur góð áhrif á líf okkar.
= Our friendship has a good influence on our life.

In English you also use “has” with “an influence” / “an effect”:

X has a good effect on Y.

Icelandic does this the same way:

að hafa áhrif á e-ð = to have an effect on something.

Using er með (literally “is with/has got”) is used for possessing objects, not for having an effect:

Ég er með bók. = I have a book (I’ve got a book).
But Ég er með áhrif á líf þitt does not mean “I have an influence on your life” in normal Icelandic.

So hefur … áhrif á is the standard pattern for this meaning.

Why is it góð áhrif and not góðar áhrif?

Because áhrif is neuter plural, and in neuter plural the adjective has no ending in the nominative/accusative:

góð áhrif = good effects / good influence
(neuter plural: adjective = góð, noun = áhrif)

If the noun were, for example, feminine plural, then you would use -ar:

góðar vinkonur = good (female) friends (fem. pl.)

But for neuter plural:

Nom. pl.: góð áhrif
Acc. pl.: góð áhrif

No -ar there, because neuter plural adjectives use the base form góð.

What exactly is áhrif? Is there a singular form like “áhrif vs áhrifi”?

Áhrif means effect(s), influence, impact.

Important points:

It is almost always used as a plural-only noun (like “scissors” or “goods” in English).
You normally talk about having áhrif (effects), not one áhrif.

In practice you say:

áhrif – (the) effects / influence
mikil áhrif – great influence / big impact
góð/slæm áhrif – good/bad effects

There is a related singular form áhrifi in some specialized or older uses, but in everyday modern Icelandic, you can think of áhrif as plural-only and just learn:

að hafa áhrif á e-ð = to have an effect on something / to influence something

What case is líf in here, and why does á take that case?

In á líf okkar, the noun líf is in the accusative.

The preposition á can govern either:

accusative, or
dative,

depending on meaning.

Roughly:

á + accusative: direction, effect on something, or more “active” sense
(onto / affecting / into / to)
á + dative: location or static position
(on / at / in a fixed place)

Here we have:

Vinátta okkar hefur góð áhrif á líf okkar.
= Our friendship has good effects on our life (it affects our life)

This is about having an effect on something, so á takes the accusative:

á líf (okkar) = on (affecting) our life.

Why is it líf and not lífum if we mean “our lives”?

Because líf is a neuter noun whose singular and plural forms look the same in nominative and accusative:

Very simplified:

Nom. sg.: líf
Acc. sg.: líf
Nom. pl.: líf
Acc. pl.: líf
Dat. pl.: lífum

So:

líf okkar can mean our life (as one shared life) or our lives, depending on context.
If you literally needed the dative plural, you’d see the -um: í lífum okkar (in our lives).

In this sentence, líf okkar is accusative, and accusative singular and accusative plural of líf are both líf, so the form doesn’t change. The idea “our lives” comes from meaning and context, not from a visible plural ending.

Could we change the word order to Vinátta okkar hefur á líf okkar góð áhrif?

This word order is possible, but it sounds more marked and less natural as a neutral statement.

Standard, neutral order:

Vinátta okkar hefur góð áhrif á líf okkar.
[subject] [verb] [direct object] [prepositional phrase]

If you say:

Vinátta okkar hefur á líf okkar góð áhrif.

you are moving góð áhrif after the prepositional phrase, which can:

put a bit of emphasis on á líf okkar, or
sound somewhat more formal or rhetorical.

For everyday, neutral Icelandic, keep:

…hefur góð áhrif á líf okkar.

How would the sentence change if I said “My friendship has a good influence on my life” instead of “our”?

You’d replace okkar (“our”) with the 1st person singular possessive forms.

The sentence would be:

Vinátta mín hefur góð áhrif á líf mitt.

Breakdown:

vinátta mín
- vinátta (fem. sg.) → possessive agrees: mín (feminine form)
líf mitt
- líf (neuter sg.) → possessive agrees: mitt (neuter form)

So:

mín = my (for feminine nouns like vinátta)
mitt = my (for neuter nouns like líf)

Why don’t we use a definite ending, like vináttan okkar or lífið okkar?

You can use the definite forms, but it slightly changes the nuance.

Without definite endings (as in the original):
- Vinátta okkar hefur góð áhrif á líf okkar.
  This can be understood as “Our friendship has a good influence on our life/lives”, quite natural and not strongly emphasizing “the” friendship vs some other.
With definite endings:
- Vináttan okkar hefur góð áhrif á lífið okkar.
  Literally: “The our friendship has good effects on the our life.”
  This sounds more like a specific, already-known friendship and life you’ve been talking about; it’s a bit more explicitly definite and can feel heavier.

In many contexts either version is understandable, but:

The indefinite base forms (vinátta okkar, líf okkar) are very natural for a general statement about your relationship and life.
The definite versions (vináttan okkar, lífið okkar) could imply “this particular friendship/life we share”, often with some prior context.

Could we say góð áhrif á okkar líf instead of góð áhrif á líf okkar? Is there a difference?

Yes, you can say á okkar líf, but the normal, neutral order is á líf okkar.

á líf okkar
= on our life / on our lives (neutral, standard).
á okkar líf
moves okkar before the noun and tends to add emphasis, something like:
- “on our life (as opposed to someone else’s)”
- or just a slightly more contrastive or marked tone.

In most neutral contexts, Icelanders would say:

góð áhrif á líf okkar.

Use á okkar líf if you really want to stress “our”.

How is vinátta okkar hefur góð áhrif á líf okkar pronounced, especially the tricky parts?

Very roughly (using an English-friendly approximation):

vinátta ≈ “VI-nah-tta”
- stress on the first syllable: VI
- á like “ow” in how, but shorter/cleaner
- tt is a strong /t/, almost like “t-h” (aspirated)
okkar ≈ “OCK-ar”
- first o like in British “lock”
- kk is a strong /k/, again often aspirated
hefur ≈ “HE-vur”
- stress on HE
- eu like “eh-oo” merging into one sound, roughly “heh-vur”
góð ≈ “goth” (with th as in this, not thin)
- the ð is a voiced th sound
áhrif ≈ “OW-riv”
- á like before (ow in how)
- hr is pronounced with a strong h before the r, like “hriv”
líf ≈ “leev”
- long í like “ee” in see
- final f here sounds more like v: /liːv/

Main points:

Stress is always on the first syllable of each word.
á is a distinct vowel (not just plain “a”).
Final f in líf is voiced → sounds like v.