Ég legg hreint lak á dýnuna áður en ég fer að sofa.

Leggja is the infinitive, meaning to lay / to put. In the sentence, the verb has to match ég meaning I, so it appears in the 1st person singular present tense:

• ég legg = I lay / I put
• þú leggur = you lay / you put
• hann / hún leggur = he / she lays / puts

So Ég legg... means I put...

This is a very common Icelandic learner question.

• leggja = to lay / to put something somewhere
  
  • it usually takes an object
• liggja = to lie / to be lying somewhere
  
  • it describes a position or state

So:

• Ég legg lakið á rúmið = I put the sheet on the bed
• Lakið liggur á rúminu = The sheet is lying on the bed

In your sentence, the speaker is actively placing the sheet, so legg from leggja is the right verb.

The direct object of leggja is normally in the accusative case. In this sentence, lak is the direct object.

However, lak is a neuter noun, and for many neuter nouns, the nominative and accusative singular look exactly the same. So even though it is accusative here, the form stays lak.

That means:

• nominative singular: lak
• accusative singular: lak

So the case is there grammatically, but you do not see a visible change on the noun itself.

Because the adjective has to agree with the noun in gender, number, and case.

Here:

• lak is neuter
• it is singular
• it is accusative
• it is indefinite

So the adjective hreinn meaning clean becomes hreint.

That gives:

• hreint lak = a clean sheet

This is also the strong adjective form, because there is no definite article before the noun.

Icelandic normally does not use an indefinite article like English a or an.

So where English says:

• a clean sheet

Icelandic simply says:

• hreint lak

The noun without the definite article is usually understood as indefinite.

Because á can take either accusative or dative, depending on meaning.

A very important pattern in Icelandic is:

• accusative after a preposition for movement toward a place
• dative for location in a place

Here, the speaker is putting the sheet onto the mattress, so there is movement toward a destination. That is why Icelandic uses the accusative:

• á dýnuna = onto the mattress

If you were describing where something already is, you would use the dative:

• Lakið er á dýnunni = The sheet is on the mattress

Dýnuna consists of:

• dýna = mattress
• -na = the definite article in this form

So dýnuna means the mattress.

More specifically, this is the feminine singular accusative definite form, because:

• dýna is feminine
• it follows á with motion, so it is accusative
• it is definite, so the article is attached to the end of the noun

Áður en means before in the sense of before something happens.

It introduces a subordinate clause:

• áður en ég fer að sofa = before I go to sleep

So the structure is:

• main action: Ég legg hreint lak á dýnuna
• time clause: áður en ég fer að sofa

This is a very common way to connect actions in Icelandic.

Because the meanings are slightly different.

• ég sef = I sleep / I am sleeping
• ég fer að sofa = I go to sleep / I start sleeping

The construction fara að + infinitive often means to begin to do something, or in this context to go and do something.

So áður en ég fer að sofa is specifically about the moment of going to bed or falling asleep, not simply the general state of sleeping.

Here, að is part of the construction fara að + infinitive.

So:

• fara að sofa = to go to sleep / to start sleeping

It helps connect the finite verb fer with the infinitive sofa.

You will see að in many Icelandic verb patterns, but its function depends on the construction. In this sentence, it belongs with fara að.

Yes. Sofa is the infinitive form, meaning to sleep.

In ég fer að sofa, the only fully conjugated verb is fer. The verb sofa stays in the infinitive because it is part of the larger verbal expression.

So the structure is:

• fer = finite verb, present tense, 1st person singular
• að sofa = infinitive phrase

Because Icelandic normally requires an explicit subject in each finite clause.

So even though English can sometimes feel more flexible in connected phrases, Icelandic usually says:

• Ég legg ... áður en ég fer ...

not something like leaving out the second ég.

Each clause has its own subject and verb:

• Ég legg
• ég fer

Because áður en introduces a subordinate clause, and in this kind of clause the normal order is:

• conjunction + subject + finite verb

So:

• áður en ég fer að sofa

This is different from Icelandic main-clause verb-second patterns.

A useful comparison:

• Main clause: Ég legg hreint lak á dýnuna
• If the time clause comes first: Áður en ég fer að sofa, legg ég hreint lak á dýnuna

Notice that when the subordinate clause comes first, the main clause then shows inversion: legg ég. But inside the subordinate clause itself, ég fer is the normal order.