Ég set bréfið í umslag og lími frímerkið efst í horninu.

Líma is the infinitive, meaning to glue / to stick.

In the sentence, the verb is conjugated for 1st person singular present to match ég (I), so it becomes lími:

• að líma = to stick, to glue
• ég lími = I stick / I glue

This is a very common Icelandic pattern with many verbs in -a:

• að tala → ég tala
• að senda → ég sendi
• að líma → ég lími

So lími means I stick / I glue.

The verb setja (to put, place) is irregular in the present tense.

Its 1st person singular present form is:

• ég set = I put

Not all Icelandic verbs follow the same pattern. Compare:

• að líma → ég lími
• að setja → ég set

So the sentence uses two different present-tense verb forms:

• ég set
• (ég) lími

That ending is the definite article attached to the noun. Icelandic usually puts the at the end of the noun instead of using a separate word.

So:

• bréf = letter

• bréfið = the letter

• frímerki = stamp
• frímerkið = the stamp

Both nouns are neuter singular, and here they are in the accusative singular definite form.

This is very normal in Icelandic. Instead of a separate word like English the, Icelandic often uses a noun ending.

Í umslag means into an envelope or in an envelope, depending on context, but here the sense is putting the letter into an envelope.

There are two separate things going on:

1. No definite article
   • umslag = envelope
   • umslagið = the envelope

So í umslag is into an envelope / in an envelope, not into the envelope.

2. Case after í The preposition í can take either:
   • accusative for movement into
   • dative for location in

Here the idea is motion: the letter is being put into the envelope, so accusative is used.

Because umslag is neuter singular, its indefinite nominative and accusative forms look the same, so you do not see a different ending here.

Yes, both are after í, but Icelandic changes case depending on whether the meaning is movement or location.

1. í umslag

This expresses movement: the letter is being put into the envelope.

• í + accusative = motion into something

2. í horninu

This expresses location: the stamp is placed in the corner, not moving into the corner in a directional sense.

• í + dative = location in something

So the contrast is:

• set bréfið í umslag = put the letter into an envelope
• frímerkið efst í horninu = the stamp is at the top in the corner

This accusative/dative distinction after certain prepositions is one of the most important grammar patterns in Icelandic.

Efst means at the top, highest up, or topmost.

It comes from efri / efstur, which relate to being higher or upper. In this sentence, efst works adverbially and tells you where the stamp goes:

• efst í horninu = at the top in the corner
• more naturally in English: in the top corner

So efst helps specify that it is not just any corner, but the upper part of it.

Because Icelandic, like English, often leaves out a repeated subject when two verbs are joined by og (and) and the subject is the same.

So:

• Ég set bréfið í umslag og lími frímerkið...

means:

• I put the letter in an envelope and (I) stick the stamp...

You could think of the second ég as understood. Repeating it would usually be unnecessary.

Because both are direct objects of verbs.

• set takes the thing being put as its object:

  • set bréfið = put the letter

• lími takes the thing being stuck/glued as its object:

  • lími frímerkið = stick the stamp

In Icelandic, direct objects are often in the accusative case.

So:

• bréfið = accusative singular definite of bréf
• frímerkið = accusative singular definite of frímerki

Since both nouns are neuter singular, the accusative definite form ends in -ið here.

It can cover both ideas.

In this kind of sentence, líma frímerkið is the normal way to say stick/glue the stamp on. English often says stick a stamp on the envelope, even if no literal glue is mentioned. Icelandic líma can naturally be used in that context.

So the verb is not strange here; it is a normal choice for attaching a stamp.

Horninu is the dative singular definite form of horn (corner, also horn in other contexts).

Breakdown:

• horn = corner / horn
• horni = dative singular indefinite
• horninu = dative singular definite = the corner

It is dative because it follows í in a location sense:

• í horninu = in the corner

Yes, but this sentence is fairly straightforward.

The basic structure is:

• Ég = subject
• set = verb
• bréfið = object
• í umslag = prepositional phrase
• og lími frímerkið efst í horninu = second coordinated verb phrase

So it is essentially:

• I put the letter into an envelope and stick the stamp in the top corner

This is a normal main-clause word order in Icelandic.

One useful thing to notice is that Icelandic often keeps adverbial/location expressions later in the clause:

• lími frímerkið efst í horninu

where efst í horninu tells you where the stamp is placed.

You mainly learn it together with the noun, because grammatical gender is something that has to be memorized.

For this word:

• umslag is neuter

A few clues can help sometimes:

• many Icelandic nouns ending in -ag or with no ending may be neuter, but this is not completely reliable
• dictionary entries usually show the gender
• you can also tell from article forms and agreement in other sentences

In this sentence, the form umslag itself does not visibly show much, but it is a neuter noun.

Yes, depending on how you phrase it.

In real usage, a stamp is often said to be on the envelope, so you may also meet sentences with á umslagið or similar wording. But the sentence you have is focusing on the position as efst í horninu—literally at the top in the corner.

So this version is perfectly natural for describing placement, and it highlights the corner rather than the surface of the envelope.

A learner should mainly notice that Icelandic can describe location from slightly different angles, just like English can say:

• on the envelope
• in the corner
• at the top corner

The Icelandic sentence chooses the corner/location perspective.