Bókasafnsfræðingurinn hjálpar mér.

Because Icelandic makes compound nouns very freely.

This word can be broken down like this:

• bók = book
• safn = collection
• bókasafn = library
• fræðingur = scholar, specialist, expert

So bókasafnsfræðingur is literally something like library specialist or library-science professional, and in normal translation it is librarian.

The -s- in the middle is very common in Icelandic compounds. It often comes from a genitive form and helps link the parts together.

That ending shows the definite article, meaning the.

The dictionary form is:

• bókasafnsfræðingur = a librarian

In the sentence, you have:

• bókasafnsfræðingurinn = the librarian

In Icelandic, the is usually attached to the end of the noun instead of being a separate word. With many masculine nouns ending in -ur, the definite nominative singular ends up looking like -urinn.

Because hjálpar is the present-tense form that matches a third-person singular subject.

The infinitive is:

• að hjálpa = to help

But here the subject is bókasafnsfræðingurinn = the librarian, which is third-person singular, so the verb becomes:

• hjálpar = helps / is helping

A useful mini-pattern is:

• ég hjálpa = I help
• þú hjálpar = you help
• hann/hún/það hjálpar = he/she/it helps
• við hjálpum = we help
• þið hjálpið = you all help
• þeir/þær/þau hjálpa = they help

Because the verb hjálpa takes the dative case.

The forms of I / me are:

• ég = I, nominative
• mig = me, accusative
• mér = me, dative

Since hjálpa uses a dative object, Icelandic says:

• hjálpa mér = help me

not hjálpa mig.

This is something you often just have to learn with the verb: hjálpa + dative.

Because the cases show the roles.

In this sentence:

• Bókasafnsfræðingurinn is the subject
• mér is the object affected by the verb

So the meaning is The librarian helps me, not I help the librarian.

English relies more on word order for this. Icelandic uses both word order and case endings, so forms like mér give you important grammatical information.

Here, yes: the most neutral order is:

• Subject + Verb + Object
• Bókasafnsfræðingurinn hjálpar mér.

But Icelandic is more flexible than English because case endings help show who is doing what.

Still, in main clauses Icelandic usually follows the verb-second pattern. That means if something else comes first, the finite verb still tends to stay in second position:

• Í dag hjálpar bókasafnsfræðingurinn mér.
  Today the librarian helps me.

So the sentence you were given is the normal, straightforward order.

It can mean either, depending on context.

Icelandic simple present often covers both:

• The librarian helps me
• The librarian is helping me

If you want to make the ongoing action especially clear, Icelandic can also use vera að + infinitive:

• Bókasafnsfræðingurinn er að hjálpa mér.
  = The librarian is helping me.

So the original sentence is perfectly natural, and context tells you how to read the present tense.

Grammatically, it is masculine.

You can tell from the noun pattern and the definite ending. Icelandic nouns belong to grammatical genders, and this one is treated as masculine.

That does not always mean the real person must be male. In Icelandic, some profession words are grammatically masculine even when referring to a woman. Context or later pronouns can make that clear.

No. It is correct, but it can sound a bit formal or professional.

Another common word is:

• bókavörður = librarian

So depending on context, you may hear either one. Roughly speaking:

• bókasafnsfræðingur can feel more like library professional / library-science specialist
• bókavörður is often a more everyday word for librarian

Both are useful to recognize.