Hún gleymir stundum frímerkinu, en aldrei umslaginu.

Because the verb gleyma (to forget) takes the dative case in Icelandic.

So:

• frímerkið = the stamp (nominative/accusative)
• frímerkinu = the stamp (dative)

and

• umslagið = the envelope
• umslaginu = the envelope (dative)

A learner often expects a direct object here, but with gleyma, Icelandic uses dative instead.

That ending shows two things at once:

• -nu marks the dative singular definite
• the nouns themselves are neuter

Both frímerki and umslag are neuter singular nouns, so in the definite dative singular they become:

• frímerkinu
• umslaginu

So the ending is not random; it is a regular grammar pattern.

In Icelandic, the is usually attached to the end of the noun as a suffix, instead of being a separate word.

So:

• frímerki = a stamp / stamp
• frímerkið = the stamp
• frímerkinu = to/for/from the stamp or simply the stamp in the dative role

The same happens with umslag → umslagið → umslaginu.

Because Icelandic, like English, often leaves out words that are easily understood from the previous clause.

So:

• Hún gleymir stundum frímerkinu = She sometimes forgets the stamp
• en aldrei umslaginu = but never the envelope

The full version would be something like:

• Hún gleymir stundum frímerkinu, en hún gleymir aldrei umslaginu.

But repeating hún gleymir would sound less natural here. The shorter version is normal.

Stundum means sometimes, and Icelandic adverbs often appear in that kind of middle position in the clause.

So:

• Hún gleymir stundum frímerkinu

is a very natural word order:
subject + verb + adverb + object

You may also see adverbs in other positions in Icelandic depending on emphasis, but this placement is common and neutral.

Gleymir is the present tense, 3rd person singular form of gleyma (to forget).

So:

• ég gleymi = I forget
• þú gleymir = you forget
• hún gleymir = she forgets

Here it matches hún (she), so hún gleymir means she forgets.

No. Hún is the feminine pronoun and can refer to:

• a female person: she
• a feminine noun, if the context is about a thing

In this sentence, it most naturally means she, referring to a woman or girl. But grammatically, Icelandic pronouns can also agree with the grammatical gender of nouns.

Because after gleyma, the noun must be in the dative, not the nominative or accusative.

Compare:

• frímerkið = the stamp (dictionary-style definite form you may first learn)
• frímerkinu = the stamp after a dative-taking verb like gleyma

So this is not a different word, just a different case form of the same noun.

Grammatically, it is singular definite:

• frímerkinu = the stamp
• umslaginu = the envelope

So the most direct reading is about a particular stamp and a particular envelope.

But in real life, context matters. Sometimes a definite singular noun can sound more general in translation, depending on the situation. Still, the Icelandic form here is clearly singular and definite.

En means but.

It connects two contrasting ideas:

• She sometimes forgets the stamp
• but never the envelope

So en introduces the contrast between stundum (sometimes) and aldrei (never).