Doordat de scanner weer werkte, kon ik het formulier toch nog op tijd opsturen.

Both doordat and omdat can often be translated as because, but they are not used in exactly the same way.

• doordat usually introduces a more direct, factual cause-and-effect relationship
• omdat more often gives a reason or explanation, including a personal reason

In this sentence, the scanner working again directly caused the result that the form could be sent on time, so doordat fits very naturally.

A rough distinction:

• Doordat de scanner weer werkte... = because the scanner was working again, as a factual cause
• Omdat de scanner weer werkte... = because the scanner was working again, giving the reason

In everyday Dutch, people do sometimes use omdat where doordat would be more precise.

Because doordat introduces a subordinate clause, and in Dutch subordinate clauses the finite verb usually goes to the end.

So:

• main clause: De scanner werkte weer
• subordinate clause: doordat de scanner weer werkte

That final verb placement is one of the most important Dutch word order patterns to learn.

That is because Dutch main clauses follow the verb-second rule.

The first position in the sentence is taken up by the entire subordinate clause:

Doordat de scanner weer werkte,

So in the main clause, the finite verb must come next:

kon ik het formulier toch nog op tijd opsturen

If the sentence started directly with ik, then you would get:

Ik kon het formulier toch nog op tijd opsturen, doordat de scanner weer werkte.

So:

• first position = Doordat de scanner weer werkte
• second position = kon
• then subject = ik

Here weer means again.

So de scanner weer werkte means the scanner was working again, or had started working again after not working.

This is a very common word in Dutch, and it often means:

• again
• back
• once more

Be careful not to confuse it with the noun het weer, which means the weather.

These two little words add important nuance.

• toch suggests something like still, nevertheless, or despite that
• nog suggests still, yet, or after all
• together, toch nog gives the idea that something was difficult or seemed unlikely, but it still happened in the end

So the sentence does not just say the speaker sent the form on time. It suggests there was some obstacle, probably the scanner not working, but despite that, the speaker still managed to send it in time.

That is why toch nog feels very natural here.

Op tijd means on time or in time, depending on context.

Here it means the form was sent before the deadline.

A useful point:

• op tijd = on time / before it is too late
• English sometimes distinguishes on time and in time, but Dutch very often uses op tijd for both ideas

So nog op tijd means still in time or still before the deadline.

Opsturen is a separable verb:

• op
  • sturen

When the verb is finite in a main clause, it often splits:

• Ik stuur het formulier op

But when it appears as an infinitive, as it does here after kon, it stays together:

• Ik kon het formulier opsturen

Because Dutch tends to place infinitives near the end of the clause, you get:

• kon ik het formulier toch nog op tijd opsturen

So the verb is:

• split when finite in many main clauses
• unsplit when infinitive, as here

Sturen means to send, but opsturen often has the more specific sense of send off, mail in, or submit by sending.

For something like a form, opsturen is very natural, because it suggests sending it off to the person or organization that needs it.

So:

• sturen = send
• opsturen = send off / submit / mail in

In many contexts, opsturen is the more idiomatic choice for documents and forms.

Dutch often uses the simple past for narration, background events, and completed situations in the past, especially in writing.

So:

• werkte = was working / worked
• kon = could / was able to

This sounds natural and straightforward in a past narrative.

Dutch could also use perfect forms in some contexts, but the simple past is very common for telling what happened:

• Doordat de scanner weer werkte, kon ik...

Yes. A few natural alternatives are possible, though the nuance may shift slightly.

For example:

• Ik kon het formulier toch nog op tijd opsturen, doordat de scanner weer werkte.
• Omdat de scanner weer werkte, kon ik het formulier toch nog op tijd opsturen.
• Doordat de scanner het weer deed, kon ik het formulier toch nog op tijd opsturen.

Notes:

• moving the doordat clause changes emphasis, but not the basic meaning
• het weer deed is a very common, idiomatic way to say a machine was working again
• using omdat instead of doordat is possible in everyday speech, though doordat is a bit more precise for cause and effect

That is simply because Dutch nouns have grammatical gender/commonness that determines whether they take de or het.

• de scanner
• het formulier

Unfortunately, this usually has to be learned with each noun. There is not always a reliable way to guess.

A useful pattern here is that many words ending in -ier, like formulier, are het words, but there are exceptions in Dutch, so it is still best to learn the article together with the noun.