Yes, hafa can also mean to have, but eiga is very commonly used for possession or having something available.
In this sentence, eiga is very natural for the idea of having enough oil as a resource or supply.
A useful rough guide is:
So Við eigum ekki næga olíu is a normal and idiomatic way to say We do not have enough oil.
In a normal Icelandic statement, ekki usually comes after the finite verb.
So:
This is standard Icelandic word order. English needs do in a sentence like we do not have, but Icelandic does not use that kind of helper verb here. It just puts ekki after the conjugated verb.
Because adjectives in Icelandic must agree with the noun they describe in:
Here, olía is:
So nægur has to appear in the feminine singular accusative form:
That is why you get:
For comparison:
Because olíu is the accusative singular form of olía.
The noun olía means oil, and in this sentence it is the direct object of the verb eiga. Many Icelandic verbs, including eiga, take an accusative object.
So:
That is why the sentence has næga olíu, not næga olía.
Because oil is being treated as a mass noun, just like in English.
When you talk about a substance in general, Icelandic often uses the singular:
If you used the plural olíur, that would usually suggest:
So in a sentence meaning We do not have enough oil, the singular is exactly what you would expect.
Because the sentence is talking about oil in general, not a specific oil already identified as the oil.
Icelandic does have a definite article, but it is usually attached to the noun as an ending rather than written as a separate word.
So:
Here, the meaning is just enough oil, not enough of the specific oil, so no definite article is used.
Yes. A very common alternative is:
This also means We do not have enough oil.
The difference is mainly in structure:
Both are natural. Learners often meet both patterns.
A useful thing to notice:
Usually, in a normal standalone sentence, you should keep við.
Even though eigum clearly shows 1st person plural, Icelandic does not regularly drop subject pronouns the way some languages do.
So the normal full sentence is:
If við is omitted, it may sound incomplete, context-dependent, or stylistically marked unless the situation makes the subject extremely obvious.
This sentence is a very useful model for several basic Icelandic patterns:
Subject + verb + ekki + object
The verb agrees with the subject
Direct objects often appear in the accusative
Adjectives agree with nouns in gender, number, and case
So this one sentence gives you practice with:
That makes it a very good sentence to study closely.