Ég trúi því sennilega ekki fyrr en ég sé svarið skrifað í tölvupóstinum.

Because trúa normally takes a dative object in Icelandic.

• að trúa einhverju = to believe something
• so því is the dative form of það

Here því means it/that, referring to the claim or information being talked about. An English speaker often expects a direct object like það, but with trúa, Icelandic uses dative instead.

Trúa is the infinitive, meaning to believe.
Trúi is the 1st person singular present tense form, used with ég.

So:

• ég trúi = I believe
• að trúa = to believe

This is a very common pattern in Icelandic: many verbs have -a in the infinitive and -i in the ég present form.

Sennilega is an adverb meaning probably.

Its placement is natural Icelandic middle-of-the-clause word order:

• Ég trúi því sennilega ekki ...

A short object pronoun like því often comes early, and sentence adverbs like sennilega commonly appear before ekki. So this word order sounds very normal.

A learner should mainly notice that Icelandic adverbs do not always sit exactly where English adverbs do.

Ekki fyrr en is a fixed expression meaning not until.

Literally, it is something like:

• not earlier than

So:

• Ég trúi því sennilega ekki fyrr en ...
• = I probably won’t believe it until ...

This is a very common Icelandic structure, and it is best learned as a chunk.

Yes, the infinitive is sjá = to see, but the present tense is irregular.

The relevant present forms are:

• ég sé = I see
• þú sérð = you see
• hann/hún sér = he/she sees
• við sjáum = we see

So ég sé is simply the normal present-tense form here. It is irregular, so it has to be memorized.

Svarið is the definite form of svar = answer.

Here it is the object of sé (I see), and sjá takes the accusative.

Because svar is a neuter noun, its definite singular nominative/accusative form is:

• svar = answer
• svarið = the answer

So the -ið is the suffixed definite article.

Skrifað is the past participle of skrifa = to write, and here it functions like an adjective meaning written.

The structure is:

• sjá + object + participle

So:

• ég sé svarið skrifað
• = I see the answer written

In other words, skrifað describes the state of svarið. It agrees with svarið, which is neuter singular, so skrifað is also in the neuter singular form.

The preposition í can take either:

• accusative for motion into
• dative for location in

Here the meaning is location:

• written in the email

So Icelandic uses the dative:

• í tölvupóstinum

Also, tölvupóstinum is the definite singular form, meaning in the email rather than just in an email.

Here tölvupóstinum most naturally means the email message.

So the phrase means:

• in the email
• or more literally in the email message

Because it has the definite ending, it suggests a specific email already known from the context.

Because this is a subordinate clause after fyrr en.

In main clauses, Icelandic usually follows the verb-second pattern. But subordinate clauses normally do not use that same inversion pattern.

So:

• main clause: Ég trúi því sennilega ekki ...
• subordinate clause: ... fyrr en ég sé svarið ...

That is why the subject ég comes before the verb sé in the second clause.

It usually refers to the whole claim or piece of information being discussed, not to a specific noun in the sentence.

So the sense is:

• I probably won’t believe it ...

where it means that claim / that statement / that information.

This is very natural in Icelandic, just as in English. You do not always need to repeat the full content again.