Learn Icelandic: Hrísgrjónin eru tilbúin eftir tíu mínútur.

In English, the word rice is an uncountable singular noun, so we say 'the rice is'. However, in Icelandic, hrísgrjón is a neuter plural noun. Because the subject is plural, you must use the plural verb eru and add the plural definite article to get hrísgrjónin.

Why does the adjective tilbúin end in -in instead of -inn?

Adjectives have to agree with the noun they describe. The dictionary form is tilbúinn, but since hrísgrjónin is a neuter plural noun in the nominative case, the adjective must change to its neuter plural nominative form, which is tilbúin.

The literal translation says 'after ten minutes', but we usually say 'in ten minutes' in English. Is eftir the standard way to express this?

Yes! When talking about something happening in a certain amount of time in the future, Icelandic uses eftir (literally 'after'). You will hear this constantly for future plans, like eftir viku (in a week).

What case is tíu mínútur in, and why?

It is in the accusative case. The preposition eftir takes the accusative case when it refers to a duration of time passing. Here, mínútur is the accusative plural form of the feminine noun mínúta.