Ég þarf að skrá mig inn áður en ég get séð skjalið.

Because þurfa usually takes að + infinitive.

So:

• Ég þarf að skrá mig inn = I need to log in
• Ég þarf að fara = I need to go
• Ég þarf að lesa = I need to read

Here að works like English to before a verb.

The verb here is the fixed expression að skrá sig inn, which means to log in / sign in.

Literally, it is something like to register oneself in.

The pronoun changes with the person:

• ég skrái mig inn = I log in
• þú skráir þig inn = you log in
• hann/hún skráir sig inn = he/she logs in

So:

• mig is correct with ég
• sig is only used for third person reflexive meaning

English usually just says log in, but Icelandic normally uses the reflexive pronoun here.

Inn is the particle that completes the meaning of the expression skrá sig inn.

On its own:

• skrá can mean register, record, or enter
• skrá sig inn specifically means log in / sign in

So inn is not just optional. It is part of the verb phrase.

This is similar to English verb-particle combinations like:

• log in
• sign in
• fill out

In Icelandic, with verb-particle expressions like this, the object pronoun often comes before the particle.

So the normal order is:

• skrá mig inn
• skrá sig inn

not usually:

• skrá inn mig

So inn stays at the end of that little verb phrase.

Because geta does not take að before the next verb.

So:

• ég þarf að sjá = I need to see
• ég get séð = I can see

That difference is important:

• þurfa → usually að + infinitive
• geta → no að

Also, after geta, Icelandic uses the special non-finite verb form seen in séð.

This is a very common learner question.

The verb is að sjá = to see, but after geta, Icelandic normally uses the form séð:

• ég get séð = I can see
• hann getur séð = he can see

So this is just the pattern of geta that you need to learn.

A few more examples:

• að lesa → ég get lesið
• að fara → ég get farið
• að finna → ég get fundið

So séð here is not random; it is the form Icelandic uses after geta.

Áður en is the conjunction meaning before when it introduces a full clause.

So:

• áður en ég get séð skjalið = before I can see the document

You can think of it as a set phrase:

• áður en
  • clause

Examples:

• Áður en ég fer... = Before I go...
• Áður en við byrjum... = Before we begin...

So in this sentence, áður en introduces the second clause.

Because Icelandic usually puts the definite article onto the noun.

The basic noun is:

• skjal = document

With the definite article:

• skjalið = the document

Here skjal is a neuter noun, and -ið is the normal definite ending here.

So:

• skjal = a document
• skjalið = the document

It is the accusative, because it is the direct object of sjá.

The verb sjá takes an accusative object:

• sjá skjal = see a document
• sjá skjalið = see the document

For this noun, the nominative and accusative singular happen to look the same:

• skjalið

So even though it is accusative here, the form does not change.

Mig is also accusative.

It is the object of skrá in the expression skrá mig inn.

Compare:

• ég = I
• mig = me

So the sentence literally contains something like:

• I need to register me in

but idiomatically it means:

• I need to log in

Yes. You can say:

Áður en ég get séð skjalið þarf ég að skrá mig inn.

That means the same thing.

Notice the word order in the main clause after the fronted phrase:

• þarf ég
• not ég þarf

That happens because Icelandic is a verb-second language in main clauses.

So both are correct:

• Ég þarf að skrá mig inn áður en ég get séð skjalið.
• Áður en ég get séð skjalið þarf ég að skrá mig inn.

Yes, it is a very normal and standard way to say it, especially in formal or neutral language.

You may also hear more casual forms influenced by English, but skrá sig inn is an excellent standard expression to learn.

Related expressions:

• skrá sig inn = log in
• skrá sig út = log out
• innskráning = login / sign-in
• útskráning = logout / sign-out