Breakdown of Hún var með gamla olíu í vélinni sinni.
vera
to be
hún
she
gamall
old
í
in / into / to (before the hour, in telling time) / per (in frequency expressions)
Questions & Answers about Hún var með gamla olíu í vélinni sinni.
Why use sinni here instead of hennar for "her"?
In Icelandic, when words like "his", "her", or "their" refer back to the subject of the sentence, you must use a form of the reflexive pronoun sinn (meaning "one's own"). Since the engine belongs to the subject (Hún), she has oil in "her own" engine, making it sinni.
How do we get the exact form sinni from sinn?
The pronoun sinn must match the noun it describes in gender, number, and case. The word for engine, vél, is feminine and singular. Because the oil is located inside it, the preposition í (in) triggers the dative case. The feminine singular dative form of sinn is sinni.
Why does the word for engine have the definite article added (vélinni) if it is followed by a possessive pronoun?
In everyday Icelandic, it is standard to keep the definite article on a noun when it is followed by a possessive pronoun. You are literally saying "in the engine her". So the dative form of vél becomes vélinni (the engine) before adding sinni.
Why does the sentence use var með instead of a past tense form of hafa (to have)?
The phrase vera með (literally "to be with") is the most common way to express physical possession or containing something in Icelandic. Using the past tense var með is the most natural way to say she physically "had" oil in the engine.