Learn Hardcore Icelandic: Vasinn er lítill.

Vasinn is the definite form of the masculine noun vasi meaning pocket. The suffix -inn acts as the definite article, so vasinn = the pocket.

Why doesn’t Icelandic use a separate word for the or a here?

Icelandic expresses definiteness by attaching a suffix to the noun instead of using a separate article. Thus vasi (no suffix) is indefinite (a pocket), while vasinn (with -inn) is definite (the pocket).

What gender, number, and case is vasinn in?

It is masculine, singular, nominative. As the subject of the sentence, the noun appears in its nominative form, and vasi is a masculine noun.

Why is the adjective lítill not lítli or lítil here?

Because it is a predicate adjective (it follows the copula er). Predicate adjectives use the strong declension, matching masculine nominative singular with -ll → lítill. Attributive adjectives (before a definite noun) use the weak declension → lítli (as in lítli vasinn).

How do I say a pocket is small (indefinite)?

Use the noun without the definite ending and the strong adjective:
Vasi er lítill.

How would I express the small pocket as a noun phrase?

Place the adjective before the noun in its weak (definite) form:
lítli vasinn = the small pocket.
For an indefinite version, use lítill vasi = a small pocket.

How do I ask Is the pocket small? in Icelandic?

Invert verb and subject:
Er vasinn lítill?

How do I negate the sentence The pocket is small?

Insert ekki (not) after the verb:
Vasinn er ekki lítill.