Í tölvupóstinum skrifa ég fyrsta stafinn með hástaf, en restina með lágstöfum.

This is because Icelandic normally follows a verb-second pattern in main clauses.

When the sentence begins with Í tölvupóstinum (In the email), that phrase takes the first position. The finite verb then comes second, so you get:

• Í tölvupóstinum skrifa ég ...
• literally: In the email write I ...

If there were no fronted phrase, the normal order would be:

• Ég skrifa fyrsta stafinn ...

So this is a very typical Icelandic word-order change, not something unusual about skrifa.

Because í takes different cases depending on meaning.

Here it means in in the sense of location, so it takes the dative:

• í tölvupósti = in an email
• í tölvupóstinum = in the email

The ending -num is the dative singular definite ending here.

Very roughly:

• tölvupóstur = an email, nominative
• tölvupóstinn = the email, accusative
• tölvupóstinum = in the email / to the email, dative depending on context

So í tölvupóstinum is correct because the sentence is talking about being in something, not motion into it.

It is a compound noun plus the definite article ending.

• tölva = computer
• póstur = mail/post
• tölvupóstur = email
• tölvupóstinum = the email in the dative singular

Icelandic often forms compounds this way, and then the definite article is attached to the end of the noun rather than written as a separate word.

So instead of a separate word for the, Icelandic usually adds it onto the noun:

• tölvupóstur = email
• tölvupósturinn = the email
• tölvupóstinum = in the email / to the email

Because this phrase is the direct object of skrifa (to write), so it is in the accusative case, not the nominative.

Compare:

• fyrsti stafurinn = the first letter as a subject
• fyrsta stafinn = the first letter as an object

Both words change:

• fyrsti → fyrsta
• stafurinn → stafinn

That is because fyrsta stafinn is masculine singular accusative, and the adjective has to agree with the noun in gender, number, and case.

Yes. In Icelandic, stafur can mean a letter of the alphabet.

So here:

• fyrsta stafinn = the first letter

This is completely normal Icelandic usage. Even though English staff or stave may look similar historically, here the meaning is specifically an alphabetic character.

Restina means the rest and is the definite accusative singular form of rest.

It is in the accusative because it is also an object of skrifa:

• skrifa fyrsta stafinn ...
• en restina ...

So the sentence is contrasting two objects:

• the first letter
• the rest

The -ina ending shows the definite form here. In English we use a separate word, the, but Icelandic attaches definiteness to the noun itself.

Because the meaning is different in each part.

• fyrsta stafinn með hástaf = you write the first letter as a capital letter
• restina með lágstöfum = you write the rest in lowercase letters

The first part refers to just one letter, so singular makes sense:

• hástaf = capital letter

The second part refers to all the remaining letters, so plural makes sense:

• lágstöfum = lowercase letters

So the number reflects the meaning:

• one first letter → singular
• many remaining letters → plural

Here með takes the dative, because it means with / by means of / using.

So:

• með hástaf
• með lágstöfum

The dative plural is easy to see in lágstöfum because of the -um ending.

With hástaf, the singular form looks the same as another case, so the form itself does not make the case obvious. But in this sentence it is understood as dative after með.

This use of með is about manner or instrument: writing something with capitals or lowercase letters.

Repeating með makes the contrast clearer and more natural:

• ... með hástaf, en restina með lágstöfum.

It pairs the two ideas neatly:

• with a capital letter
• with lowercase letters

You may sometimes see prepositions omitted in coordinated structures when the meaning is very clear, but here repeating með is the most natural and standard choice.

It is an ordinal numeral, which behaves like an adjective.

So fyrsta means first, and it agrees with the noun it modifies:

• gender
• number
• case

Here it matches stafinn, which is masculine singular accusative, so the correct form is fyrsta.

This is why you should think of Icelandic ordinals much like adjectives in grammar, even though in meaning they are numbers.

Yes, that would be grammatically possible, but it would sound different in emphasis.

• Í tölvupóstinum skrifa ég ... puts focus on in the email
• Ég skrifa fyrsta stafinn ... starts more neutrally with I
• putting í tölvupóstinum later may make it sound less prominent

Because Icelandic allows some flexibility in word order, moving elements around often changes emphasis more than basic meaning. The version in your sentence highlights the context first: in the email.