Ég geri ekki allt í einu.

Because geri is the 1st person singular present tense form of the verb að gera (to do / to make).

So:

• ég geri = I do
• þú gerir = you do
• hann/hún/það gerir = he/she/it does
• við gerum = we do
• þið gerið = you (plural) do
• þeir/þær/þau gera = they do

This is a very common verb, and it changes stem in the singular forms, so you cannot just use the infinitive gera after ég.

In a normal main clause, ekki usually comes after the finite verb.

So:

• Ég geri ekki ... = I do not do ...

This is the standard position for sentence negation in Icelandic.
Compare:

• Ég kem ekki. = I am not coming.
• Hún veit ekki. = She does not know.

So Ég ekki geri... would be wrong in ordinary neutral word order.

Here allt means everything or all of it.

It is the neuter singular form of allur (all), but in this sentence it is being used on its own, not directly before a noun.

So:

• allt = everything / it all

Examples:

• Ég borða allt. = I eat everything.
• Hann veit allt. = He knows everything.

In your sentence, allt is the object of geri.

Because this is a fixed expression: í einu.

The preposition í can take different cases, and in this expression it uses the dative, so einu is the dative form of einn (one).

So literally:

• í einu = in one

But idiomatically it means:

• at once
• in one go
• all at the same time

You do not normally change this to í eitt in this phrase.

Not always. Allt í einu is a very common Icelandic expression that can also mean all of a sudden / suddenly.

So the phrase has two common uses:

• allt í einu = all at once
• allt í einu = suddenly

Context tells you which meaning is intended.

In Ég geri ekki allt í einu, the natural meaning is I don’t do everything all at once / in one go, not suddenly.

Yes, it is natural.

It suggests something like:

• I don’t do everything at once
• I don’t do it all in one go
• I do things step by step

So it sounds like the speaker is saying they prefer to do things gradually rather than all together.

Yes, but the given order is the normal neutral order:

• Ég geri ekki allt í einu.

You can move parts of the sentence for emphasis, but that makes it more marked. For example:

• Allt geri ég ekki í einu.

This is possible, but it sounds more emphatic, like Everything, I don’t do all at once.

For learners, the safest and most natural version is the original one.

A rough English-style approximation is:

• yeg GEH-rɪ EH-kih alht ee AY-nʏ

A few helpful notes:

• Ég begins with a sound close to yehg
• g in geri is a hard g
• kk in ekki is pronounced distinctly
• ll in allt is not pronounced exactly like English ll
• í is a long ee
• einu sounds roughly like AY-nu / AY-nʏ

If you want to sound more natural, it is best to listen to native audio, because Icelandic pronunciation is not always obvious from spelling.

Normally you should include ég.

Unlike some languages where subject pronouns are often dropped, Icelandic usually keeps the subject pronoun in ordinary sentences:

• Ég geri ekki allt í einu.

Leaving out ég would usually only happen in special contexts, such as diaries, notes, coordinated clauses, or very informal speech. For a learner, it is best to include it.

The sentence breaks down like this:

• Ég = subject (I)
• geri = finite verb (do)
• ekki = negation (not)
• allt = object (everything / it all)
• í einu = adverbial phrase (at once / in one go)

So the structure is:

• Subject + verb + not + object + adverbial phrase

That is a very useful pattern in Icelandic main clauses.