Learn Hardcore Icelandic: Ég á átta penna.

Syntactic context: It appears with a subject ( ég ) and takes a direct object ( penna ). Prepositions never take subjects.
Morphology: As a verb, á is finite (present tense of eiga ) and sits in V2 position. As a preposition, it would govern a noun directly (e.g. á borðið “on the table”) but wouldn’t allow a subject pronoun to precede it as part of the same clause.

hafa is used as an auxiliary verb (“have done something”) or for more abstract senses of “have” (e.g. “to hold inside,” or “to have to” when paired with að).
To say “I have pens” in the sense “I own pens,” you always use ég á penna, not ég hef penna.

Why is the pronoun ég necessary here? Can it be dropped?

The form á is formally identical for both 1st person singular (“I own”) and 3rd person singular (“he/she/it owns”). Because of that ambiguity, Icelandic speakers normally include ég (“I”) to make clear who the subject is.

Unlike some other verbs whose endings differ for 1st vs. 3rd person, á does not, so the pronoun stays.

Why is the noun penni in the form penna and not penni, penna, or penna with a different case?

penni (“pen”) is a masculine i-stem noun. Its principal forms are:
• Singular: N penni, A penna, D penna, G penna
• Plural: N penna, A penna, D pennum, G penna

In Ég á átta penna the noun is the direct object of á, so it must be in the accusative. The accusative plural of is . That same form also appears as genitive singular, nominative plural and genitive plural, which can be confusing—here it’s the plural accusative.