Ég geymi penna í bókasafninu.

I store pens in the library.

Breakdown of Ég geymi penna í bókasafninu.

• ég: I
• penni: the pen
• í: in
• bókasafn: the library
• geyma: to store

Questions & Answers about Ég geymi penna í bókasafninu.

Why is penna used instead of penni, and what case is it?

Because penna is the accusative form of the noun penni (“pen”). Icelandic marks direct objects with the accusative case. In this weak masculine -i-noun class, both the singular and the indefinite plural accusative happen to be penna, so you need context (or extra words) to tell if it means “a pen” (sg.) or “pens” (pl.).

How would I make it absolutely clear that I mean just one pen or several pens?

– To force singular: say einn penna (“one pen”) or use the definite singular pennann (“the pen”).
– To force plural: use a numeral, e.g. tvo penna (“two pens”), or use the definite plural accusative pennana (“the pens”).

Why is bókasafninu in that form, and how do I recognize its case and definiteness?

The preposition í for a static location takes the dative case. The base noun is bókasafn (“library”). Its indefinite dative is bókasafni. Icelandic shows the definite article by a suffix: you attach -ið (the) plus the dative ending -u, giving bókasafninu – literally “library-the-to-me” in form, but idiomatically “in the library.”

When would í take the accusative instead of the dative?

Use í + dative for being inside something (static location). Use í + accusative for motion into something. For example:
• Ég er í bókasafninu. (“I am in the library.”) – dative
• Ég fer í bókasafnið. (“I’m going into the library.”) – accusative

What does the verb geyma mean, and what structure does it follow?

Geyma means “to store,” “to keep,” or “to save.” It’s a transitive verb: it takes an accusative direct object (what you store) and often a prepositional phrase (where you store it), as in geyma penna í bókasafninu.

Why is the sentence ordered Ég geymi penna í bókasafninu, and can I change it?

Icelandic is a V2 (verb-second) language: the finite verb must occupy the second position in the clause. Here Ég is first, geymi is second, then the object and the place phrase follow. You can emphasize another element by fronting it, but you still keep the verb in second place. For example:
• Í bókasafninu geymi ég penna. (Emphasizing “in the library.”)

How would I say “I keep the pens in the library” (with the pens)?

Make the object definite by using the definite plural accusative pennana. The sentence becomes:
Ég geymi pennana í bókasafninu.

How do I turn this into a yes/no question, “Do you keep pens in the library?”

In Icelandic yes/no questions you invert the verb and subject. With þú it becomes:
Geymir þú penna í bókasafninu?