Hástafurinn í nafninu hennar er ekki réttur.

Hástafurinn means the capital letter or the uppercase letter.

It is made from:

• hástafur = capital letter
• -inn = the suffixed definite article, meaning the

So Icelandic often puts the at the end of the noun instead of using a separate word.

Because Icelandic usually expresses definiteness by attaching the article to the noun.

So:

• hástafur = a capital letter
• hástafurinn = the capital letter

This is very common in Icelandic. The same thing happens with nafninu later in the sentence:

• nafn = name
• nafninu = the name in the dative singular

It is the subject of the sentence — the thing being talked about.

The basic structure is:

• Hástafurinn = the subject
• er = is
• ekki réttur = not correct

Since subjects are normally in the nominative in Icelandic, hástafurinn is nominative singular.

Because í takes different cases depending on meaning.

Here it means in in a location/state sense, so it takes the dative:

• í nafninu hennar = in her name

If there were movement into something, Icelandic would often use the accusative instead.

So here:

• nafninu is the dative singular definite form of nafn

No. Í can take either dative or accusative.

A useful rule is:

• dative for location or position: in
• accusative for movement into something: into

In this sentence, the capital letter is located in her name, so Icelandic uses the dative:

• í nafninu hennar

Because hennar works like a genitive possessive form meaning her, and Icelandic very often places hans, hennar, and þeirra after the noun.

So:

• nafnið hennar = her name
• bíllinn hans = his car

This word order is normal and natural in Icelandic.

Because it agrees with hástafurinn, not with nafninu.

The subject is:

• hástafurinn = masculine singular nominative

So the predicate adjective must match it:

• réttur = masculine singular nominative

Even though nafninu is nearby, it is inside the prepositional phrase í nafninu hennar and is not the word controlling the adjective.

Because this adjective is being used as a predicate adjective after er.

In Icelandic, predicate adjectives are normally in the strong form:

• Hástafurinn er réttur. = The capital letter is correct.

The weak form, such as rétti, is typically used when the adjective comes directly before a definite noun:

• rétti hástafurinn = the correct capital letter

So after er, réttur is the right form.

Because in a normal Icelandic main clause, ekki usually comes after the finite verb.

So:

• er ekki réttur = is not correct

This is standard Icelandic word order. English puts not after is too, so this part is fairly similar.

It is a standard and precise way to say it.

You may also hear:

• stór stafur = literally big letter

But hástafur is the clearer grammatical term for uppercase / capital letter. Its opposite is:

• lágstafur = lowercase letter

Yes, but the given sentence is the most neutral and straightforward order.

For example, you could move the prepositional phrase for emphasis:

• Í nafninu hennar er hástafurinn ekki réttur.

That still means essentially the same thing, but it sounds more marked, as if you are especially focusing on in her name.

The original sentence is the most natural default word order for a learner to use.