Hvolpurinn sefur rétt áður en hundaþjálfarinn kemur.

-inn is the definite article attached to the end of the noun. So:

• hvolpur = puppy
• hvolpurinn = the puppy

and

• hundaþjálfari = dog trainer
• hundaþjálfarinn = the dog trainer

Unlike English, Icelandic very often puts the onto the noun itself instead of using a separate word.

Icelandic makes a lot of compound nouns, and they are usually written as one word.

So hundaþjálfari is made up of:

• hunda- = of dogs / dog-
• þjálfari = trainer

Together, it means dog trainer.

The hunda- part comes from hundur (dog), specifically the genitive plural form hunda. Icelandic often uses this kind of form inside compounds. So even though the whole word means dog trainer, the first part literally looks like dogs'.

Both are in the nominative singular, because each one is the subject of its clause:

• Hvolpurinn sefur = The puppy sleeps
• hundaþjálfarinn kemur = the dog trainer comes

A common point of confusion is that hunda- inside hundaþjálfarinn looks like some other case. But that only affects the first part of the compound. The whole noun hundaþjálfarinn is still nominative here.

rétt áður en means just before.

The parts are:

• áður en = before
• rétt = right / just

Here rétt does not mean the opposite of left. It is an intensifier, like English right before or just before.

So:

• rétt áður en hundaþjálfarinn kemur = just before the dog trainer comes

Because áður en is the expression used when before is followed by a whole clause.

Here, what follows is a clause:

• hundaþjálfarinn kemur = the dog trainer comes

So Icelandic uses áður en.

Compare:

• áður = earlier / before
• áður en hann kemur = before he comes

So en is needed because a full subject + verb follows it.

It can mean either, depending on context.

Icelandic does not have a separate everyday form exactly like the English is sleeping. The simple present often covers both:

• sleeps
• is sleeping

So Hvolpurinn sefur could mean:

• The puppy sleeps
• The puppy is sleeping

The larger context tells you which is intended.

Because Icelandic, like English, often uses the present tense in time clauses for something that is still in the future.

English does this too:

• before he comes not usually
• before he will come

So Icelandic says:

• áður en hundaþjálfarinn kemur

even if the trainer has not arrived yet. This is completely normal.

They are both 3rd person singular present tense forms:

• sefur = from sofa (to sleep)
• kemur = from koma (to come)

So:

• hann sefur = he sleeps / is sleeping
• hann kemur = he comes / is coming

Both verbs are somewhat irregular compared with the infinitive, so they are worth memorizing as common forms.

The sentence has a very normal structure:

• Hvolpurinn sefur = main clause
• rétt áður en hundaþjálfarinn kemur = time expression with a subordinate clause

Inside the subordinate clause, Icelandic keeps the ordinary pattern:

• hundaþjálfarinn kemur = the dog trainer comes

If you move the time phrase to the front, Icelandic usually puts the verb second in the main clause:

• Rétt áður en hundaþjálfarinn kemur, sefur hvolpurinn.

That verb-second pattern is a very important feature of Icelandic word order.

It could be either.

Without more context, Hvolpurinn sefur rétt áður en hundaþjálfarinn kemur can mean:

• a habitual action: the puppy usually sleeps just before the trainer comes
• a specific present situation: the puppy is sleeping just before the trainer comes

Icelandic often leaves that distinction to context, just as English sometimes does.