Zodra ik eraan begin, wil ik het liefst meteen doorgaan tot het af is.

Zodra means as soon as.

It introduces a subordinate clause, so the conjugated verb goes to the end of that clause:

• Zodra ik eraan begin
• literally: as soon as I start on it

It is stronger than just when because it suggests immediacy.

Because zodra creates a subordinate clause.

In Dutch subordinate clauses, the conjugated verb usually goes to the end:

• main clause: Ik begin eraan
• subordinate clause: zodra ik eraan begin

This is one of the most important Dutch word-order patterns to learn.

Eraan is a pronominal adverb: er + aan.

It is used because the verb is beginnen aan something:

• beginnen aan een boek
• beginnen aan het werk
• beginnen aan iets

When the thing has already been mentioned or is understood, Dutch often replaces aan het/aan dat/aan iets with eraan:

• Ik begin aan het project → Ik begin eraan

So eraan here means something like on it or at it, depending on natural English wording.

Because the sentence begins with the subordinate clause Zodra ik eraan begin.

After that whole clause, the main clause still follows the normal Dutch verb-second rule. So the conjugated verb comes first in the main clause, before the subject:

• Zodra ik eraan begin, wil ik ...

If the sentence started directly with the subject, it would be:

• Ik wil het liefst meteen doorgaan ...

Het liefst means preferably, most of all, or ideally.

It belongs to the pattern:

• graag = gladly / like to
• liever = rather
• het liefst = preferably / most preferably

So:

• Ik wil het liefst meteen doorgaan
  = I would prefer to continue straight away

The het here is part of a fixed expression. It does not refer to the same thing as the later het in het af is.

Because they do slightly different jobs.

• Zodra tells you when the starting happens
• meteen tells you how quickly the continuing happens after that

So the sentence is not repetitive. It means something like:

• Once I start, I want to continue immediately

The idea is: starting triggers an immediate desire to keep going.

Here doorgaan means to continue or to keep going.

So:

• meteen doorgaan = continue right away
• doorgaan tot ... = continue until ...

It is a very common verb in Dutch.

Yes, this type of verb often splits when conjugated, but it stays together in the infinitive.

So you get:

• infinitive: doorgaan
• with a modal: Ik wil doorgaan
• finite form: Ik ga door

That is normal for many Dutch separable verbs.

Because willen is a modal verb, and modal verbs take a bare infinitive in Dutch.

So you say:

• Ik wil doorgaan
• not Ik wil te doorgaan

Compare:

• Ik probeer door te gaan → with proberen, you do need te
• Ik wil doorgaan → with willen, you do not

Yes. Tot and totdat can both mean until when followed by a clause.

So these are both possible:

• tot het af is
• totdat het af is

Totdat is a little more explicit and may feel slightly clearer to learners, but tot is also natural.

Because it introduces a subordinate clause, the verb goes to the end:

• tot het af is

Here af means finished, completed, or done.

Dutch often uses af zijn for something that is complete:

• Het is af = It is finished
• Wanneer is het af? = When is it finished?

So tot het af is means until it is finished.

This is a very common Dutch pattern, even though it may feel unusual to English speakers.

Yes, klaar is also possible in many situations:

• tot het klaar is

Both af and klaar can mean finished, but af is especially common for something you are working on and completing.

Very roughly:

• af = finished/completed
• klaar = ready/finished

In this sentence, af sounds very natural.

They are doing different jobs.

1. het liefst
   Here het is just part of the fixed expression het liefst.

2. tot het af is
   Here het is a real pronoun meaning it. It refers to the thing you started.

So even though they look the same, they are not functioning the same way.