Hún er orðin fullorðin, en hún heimsækir mömmu sína samt oft.

Er orðin is a very common way to say has become / is now in Icelandic.

• verða = to become
• past participle: orðinn / orðin / orðið
• with vera (to be), it gives a resultative meaning: has become, is now

So:

• Hún verður fullorðin = She becomes / will become an adult
• Hún er orðin fullorðin = She has become an adult / She is grown up now

In this sentence, the speaker is emphasizing the present result: she is now an adult.

They are two different words with different jobs.

• orðin = become, from verða
• fullorðin = grown-up / adult, from the adjective fullorðinn

So:

• er orðin = has become
• fullorðin = adult / grown up

Put together:

• Hún er orðin fullorðin = She has become grown up / She is grown up now

They look similar because they both happen to have feminine forms ending in -in.

Because the subject is hún (she), which is feminine singular.

The participle agrees with the subject:

• masculine: orðinn
• feminine: orðin
• neuter: orðið

So:

• Hann er orðinn þreyttur = He has become tired
• Hún er orðin þreytt = She has become tired
• Það er orðið kalt = It has become cold

Here, orðin matches hún.

For the same reason: it describes hún, so it agrees with the subject in gender and number.

The adjective fullorðinn changes by gender:

• masculine: fullorðinn
• feminine: fullorðin
• neuter: fullorðið

So:

• Hann er fullorðinn
• Hún er fullorðin
• Barnið er fullorðið would be grammatically possible, though not a very natural real-life sentence

In your sentence, both orðin and fullorðin agree with hún.

Yes.

• Hún er fullorðin = She is an adult / She is grown up
• Hún er orðin fullorðin = She has become an adult / She is grown up now

The version with orðin adds a sense of change or transition. It highlights that she was not always grown up, but now she is.

So the sentence as written has a slightly stronger contrast with the second half: even though she is grown up now, she still visits her mother often.

Because heimsækja (to visit) takes a direct object in the accusative case, and mamma changes in the accusative singular to mömmu.

So:

• nominative: mamma = mom
• accusative: mömmu = mom as a direct object

Examples:

• Mamma kemur = Mom is coming
• Ég heimsæki mömmu = I visit mom

In your sentence, mömmu is the thing being visited, so accusative is required.

Because sína is a reflexive possessive, and it refers back to the subject of the clause.

Here the subject is hún, and the mother belongs to that same hún, so Icelandic normally uses sinn / sín / sitt.

• hún heimsækir mömmu sína = she visits her own mother

If you said mömmu hennar, it would usually mean her mother belonging to some other female person, not the subject.

So the contrast is:

• Hún heimsækir mömmu sína = She visits her own mother
• Hún heimsækir mömmu hennar = She visits her mother (someone else’s mother)

This is one of the most important differences between Icelandic and English, since English her can mean both.

Because sinn has to agree with the noun it belongs to, not with the owner.

It describes mömmu, and mömmu is:

• feminine
• singular
• accusative

So the correct form is sína.

Compare:

• pabba sinn = his/her own dad
  
  • pabba is masculine accusative singular, so sinn
• mömmu sína = his/her own mom
  
  • mömmu is feminine accusative singular, so sína
• hús sitt = his/her own house
  
  • hús is neuter, so sitt

Here samt means still, nevertheless, or all the same.

The sentence has a contrast:

• Hún er orðin fullorðin = she is grown up now
• en hún heimsækir mömmu sína samt oft = but she still visits her mom often

So samt signals something like:

• even so
• despite that
• still

It is a very common word in contrastive sentences.

Because en joins two separate clauses, and each clause normally needs its own subject.

So we have:

1. Hún er orðin fullorðin
2. en hún heimsækir mömmu sína samt oft

In English, you also normally repeat the subject:

• She is grown up, but she still visits her mom often

You would not usually omit the second hún here.

Because it describes a habitual action: something she does often.

• heimsækir = visits
• oft = often

Together, heimsækir ... oft means visits ... often.

So the sentence is not about one single visit. It means this is a regular pattern.

Yes, Icelandic adverb placement is somewhat flexible, though some orders sound more natural in a given context.

The sentence you have:

• hún heimsækir mömmu sína samt oft

is natural and clear: she still visits her mom often.

You may also hear other placements, for example:

• hún heimsækir samt mömmu sína oft
• hún heimsækir mömmu sína oft samt — much less natural in ordinary neutral style

In general:

• samt often appears where it clearly marks the contrast
• oft usually stays close to the verb phrase it modifies

For a learner, the given word order is a good one to remember.

Yes.

• mamma = mom / mum, informal and very common in everyday speech
• móðir = mother, more formal or neutral in certain contexts

So:

• mömmu sína sounds natural and everyday
• móður sína would sound more formal or literary

Both are correct Icelandic, but they are used in different styles.

A useful way to break it down is:

• Hún = she
• er orðin = has become / is now
• fullorðin = grown up / adult
• en = but
• hún heimsækir = she visits
• mömmu sína = her own mother
• samt = still / nevertheless
• oft = often

So the grammar is basically:

subject + resultative expression, but + subject + verb + object + adverbs

This is a very typical Icelandic sentence structure and contains several important patterns learners see often:

• agreement with gender
• accusative object
• reflexive possessive sinn
• contrast with en
• adverbs such as samt and oft