Ef við eigum hveiti heima, getum við gert pítsu á morgun.

This is because of Icelandic verb-second word order in main clauses.

The sentence starts with the subordinate clause Ef við eigum hveiti heima. After that, the main clause begins, and in Icelandic the finite verb usually comes first in the main clause if something else has already taken the first position.

So:

• Við getum gert pítsu á morgun. = We can make pizza tomorrow.
• Ef við eigum hveiti heima, getum við gert pítsu á morgun. = If we have flour at home, we can make pizza tomorrow.

That inversion is very normal in Icelandic.

No. Eigum here is just the regular present tense, 1st person plural form of eiga:

• ég á
• þú átt
• hann/hún/það á
• við eigum
• þið eigið
• þeir/þær/þau eiga

After ef meaning if, Icelandic often uses the indicative for a real or possible condition, just like here.

So Ef við eigum hveiti heima means something like If we do have flour at home / If we have flour at home.

If the sentence were more hypothetical or less real, different forms could appear, for example:

• Ef við ættum hveiti heima, gætum við gert pítsu.
• If we had flour at home, we could make pizza.

Both verbs can relate to having, but they are not used in exactly the same way.

In this sentence, eiga is very natural because it suggests possessing or having available something at home. So við eigum hveiti heima means that there is flour in our possession / in the house.

Very roughly:

• eiga = own, possess, have available
• hafa = have, hold, keep, experience, be with

A learner should not try to force a perfect one-word English match. The important thing is that eiga is a normal and idiomatic choice here.

Because Icelandic has no indefinite article. There is no separate word equivalent to English a/an.

So:

• pítsa can mean pizza or a pizza, depending on context
• pítsan means the pizza

In this sentence, pítsu means a pizza, even though there is no separate word for a.

The same idea applies to many nouns in Icelandic.

Because pítsu is the accusative singular form, and gera takes a direct object in the accusative.

The basic dictionary form is pítsa. But in the sentence, pizza is the thing being made, so it appears as the object:

• pítsa = nominative
• pítsu = accusative

So:

• Pítsa er góð. = Pizza is good.
• Við gerum pítsu. = We make a pizza.

This is a very common pattern with feminine nouns of this type.

Because different nouns decline differently.

Hveiti is a neuter noun, and in this case its nominative and accusative forms are the same. So even though it is also functioning as an object after eigum, its form does not visibly change.

So the difference is not that one noun has a case and the other does not. Both are in the structure required by the sentence, but only pítsa shows a visible change:

• hveiti → hveiti
• pítsa → pítsu

This is very normal in Icelandic declension.

Heima means at home.

It is an adverb, so Icelandic does not need a separate preposition here. English uses two words, at home, but Icelandic uses the single word heima.

A very useful set to remember is:

• heima = at home
• heim = home, to home, homewards
• heiman = from home

So in this sentence, hveiti heima means flour at home.

Because after the modal verb geta meaning can / be able to, Icelandic normally uses the bare infinitive.

So:

• geta gert = can make / can do
• not geta að gert

This is similar to English, where we say:

• can make
• not can to make

So getum við gert is exactly what you should expect.

Á morgun is a very common Icelandic expression meaning tomorrow.

It is best learned as a set phrase. Word-for-word, á often means on, but you should not translate it too literally every time.

Other time expressions also work idiomatically in Icelandic, for example:

• í dag = today
• í kvöld = tonight
• á morgun = tomorrow

So here á morgun simply means tomorrow.

Yes, gera pítsu is natural and correct.

The verb gera is very common and often means do or make, depending on context. In this sentence it means make.

You may also hear:

• búa til pítsu = make a pizza

That is also natural. But gera pítsu is perfectly good Icelandic.

A helpful breakdown is:

• Ef = if
• við eigum = we have
• hveiti heima = flour at home
• getum við = can we / we can
• gert pítsu = make a pizza
• á morgun = tomorrow

So the grammar pattern is:

Ef + clause, finite verb + subject + rest of main clause

That pattern is extremely common in Icelandic:

• Ef það rignir, förum við ekki.
• If it rains, we are not going.

So this sentence is a very good example of both conditional clauses and normal main-clause inversion in Icelandic.