Ég þarf að hlaða niður tveimur skrám áður en ég get lesið skjalið.

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Breakdown of Ég þarf að hlaða niður tveimur skrám áður en ég get lesið skjalið.

• lesa: to read
• áður en: before
• tveir: two

Questions & Answers about Ég þarf að hlaða niður tveimur skrám áður en ég get lesið skjalið.

Why is there an að after þarf?

In Icelandic, many verbs that are followed by another verb take að + infinitive, similar to English to + verb.
So Ég þarf að hlaða... literally corresponds to I need to download....
Without að, it would sound ungrammatical in standard Icelandic for this meaning.

Is hlaða niður one verb or two words? How does it work?

It’s a verb + particle construction: hlaða (load) + niður (down). Together, hlaða niður means to download.
Important: niður is a particle/adverb, not a prefix stuck onto the verb in writing, so it usually remains a separate word:

• að hlaða niður = to download
  In some contexts (especially with objects/pronouns), word order can vary, but the basic idea is the same: download.

Why is it tveimur skrám and not tvær skrár?

Because hlaða niður normally takes its direct object in the dative case in Icelandic.
So two files must be in dative plural:

• nominative: tvær skrár (two files)
• dative: tveimur skrám (to/for two files, used here as the object of hlaða niður)

This is a common thing in Icelandic: some verbs govern dative objects where English uses a plain object.

How do numerals decline here? Why tveimur?