Ég þarf tvær heimildir fyrir ritgerðina, ekki bara eina.

Because heimild is a feminine noun, and the number two changes form to match gender.

The forms are:

• tveir = masculine
• tvær = feminine
• tvö = neuter

So:

• tvær heimildir = two sources

This is one of the things English speakers often notice quickly: in Icelandic, small numbers like one, two, three, and four usually agree with the noun in gender and case.

Here, heimildir means sources, especially in an academic sense: sources you use for research, references, or supporting material in a paper.

The singular is:

• heimild = source, reference, authority, or permission depending on context

In this sentence, because it is about ritgerð (essay), the academic meaning sources/references is the natural one.

It is in the accusative plural.

That is because þurfa (to need) takes a direct object, and that object is normally in the accusative.

So in:

• Ég þarf tvær heimildir
  I need two sources

the thing being needed, tvær heimildir, is the accusative object.

A useful point here is that heimildir looks the same in nominative and accusative plural, so the clearest sign of the case is actually tvær, which fits the feminine plural here.

Þarf means need or am needing here, and it comes from the verb þurfa = to need.

So:

• ég þarf = I need
• þú þarft = you need
• hann/hún/það þarf = he/she/it needs

So the sentence begins very simply:

• Ég þarf = I need

Fyrir ritgerðina means for the essay.

Here fyrir means for, in the sense of intended for or to use for. So:

• heimildir fyrir ritgerðina = sources for the essay

This is a very natural way to say that the sources are needed for writing that essay.

Also, ritgerðina is a specific essay, not just any essay, which is why it is definite: the essay.

Because it is the definite accusative singular form of ritgerð.

The base noun is:

• ritgerð = essay

But here Icelandic needs the form meaning the essay, and after fyrir in this sentence the accusative is used, so you get:

• ritgerðina = the essay (accusative singular)

So the ending -ina is part of the definite form of this feminine noun.

Because eina agrees with an omitted feminine noun, namely heimild.

The full version would be:

• ekki bara eina heimild = not just one source

But Icelandic can leave out the noun when it is obvious from context, just like English can say:

• not just one

Since the missing noun is heimild, which is feminine, one has to be in the feminine accusative singular form:

• ein = nominative feminine singular
• eina = accusative feminine singular

Because þurfa takes an accusative object, eina is the right form here.

Yes, exactly.

It is a contrast structure:

• Ég þarf tvær heimildir = I need two sources
• ekki bara eina = not just one

So the whole sentence emphasizes that one source is not enough.

A fuller version would be:

• Ég þarf tvær heimildir, ekki bara eina heimild.

But leaving out the repeated noun is more natural.

Because the second part adds a contrast or correction-like emphasis:

• I need two sources, not just one.

The comma helps separate the main statement from the clarifying contrast.

In speech, there would usually be a slight pause before ekki bara eina. So the punctuation works much like it does in English.

Yes, that would also be a reasonable translation depending on context.

Possible English renderings include:

• I need two sources for the essay, not just one.
• I need two references for the essay, not just one.

Sources is usually the most natural general translation in academic English, but references may also fit in some contexts. The Icelandic heimild covers the idea of a source you rely on or cite.

Yes, it is completely normal.

The structure is:

• Ég = subject
• þarf = verb
• tvær heimildir = object
• fyrir ritgerðina = prepositional phrase
• ekki bara eina = contrasting add-on

So it is a straightforward main clause with a very natural emphasis at the end. Nothing unusual is happening with the word order here.