Wenn du auf die blaue Schaltfläche klickst, kannst du den Download sofort starten.

Because the sentence begins with a wenn clause, and wenn introduces a subordinate clause. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

Wenn du auf die blaue Schaltfläche klickst

has this structure:

• Wenn = if / when
• du = you
• auf die blaue Schaltfläche = on the blue button
• klickst = click

This is very normal German word order after wenn.

In a main clause, German normally puts the conjugated verb in second position.

The full sentence starts with the wenn clause:

Wenn du auf die blaue Schaltfläche klickst, ...

That whole clause counts as position 1. So the next element, the main-clause verb, must come immediately after it:

..., kannst du den Download sofort starten.

This is called verb-second word order.

Compare:

• Du kannst den Download sofort starten.
• Wenn du auf die blaue Schaltfläche klickst, kannst du den Download sofort starten.

Both are correct; the second one just begins with a subordinate clause.

Wenn can mean both if and when, depending on context.

In this sentence, it can feel like either:

• If you click on the blue button, you can start the download immediately.
• When you click on the blue button, you can start the download immediately.

In instructions like this, English often prefers if or when depending on tone, but German commonly uses wenn in both kinds of situations.

The verb klicken is often used with auf when talking about clicking on something:

• auf einen Link klicken = to click on a link
• auf die Schaltfläche klicken = to click on the button

So auf is the normal preposition here.

German treats this as a fixed pattern: auf etwas klicken.

Because after auf with klicken, German usually uses the accusative here.

So:

• die Schaltfläche = the button
• auf die Schaltfläche klicken = click on the button

The adjective ending also matches the accusative feminine form:

• nominative: die blaue Schaltfläche
• accusative: die blaue Schaltfläche

For feminine singular with die, nominative and accusative look the same, so you do not see a change in the article here.

Schaltfläche means button, especially a button on a screen or in software.

It is a common word in technical or formal computer language. In everyday speech, people may also say:

• Button
• sometimes even Knopf, depending on context

So die blaue Schaltfläche is a slightly more formal or interface-style way to say the blue button.

Because Download is the direct object of starten, so it is in the accusative case.

The noun is masculine:

• nominative: der Download
• accusative: den Download

So:

• Der Download startet. = The download starts.
• Du startest den Download. = You start the download.

In your sentence, du is the subject and den Download is the thing being started.

Yes, but the meaning changes slightly.

den Download starten means:

• to start the download

It focuses on beginning the process.

etwas herunterladen means:

• to download something

It refers to downloading the file itself.

So these are not exactly the same:

• Du kannst den Download sofort starten. = You can start the download immediately.
• Du kannst die Datei sofort herunterladen. = You can download the file immediately.

The original sentence sounds very natural in a software or website context.

Sofort means immediately or right away.

In this sentence:

..., kannst du den Download sofort starten.

it comes before starten, which is a very natural position.

You may also see similar word order variations depending on emphasis, for example:

• Du kannst den Download sofort starten.
• Du kannst sofort den Download starten.

The first version is the more neutral and natural one here.

Because in German, a subordinate clause introduced by wenn is separated from the main clause by a comma.

So:

Wenn du auf die blaue Schaltfläche klickst, kannst du den Download sofort starten.

This comma is required in standard German.

Yes. Du is the informal you.

In software, websites, and instructions, German may use either:

• du for a friendly, direct tone
• Sie for a formal tone

A formal version would be:

Wenn Sie auf die blaue Schaltfläche klicken, können Sie den Download sofort starten.

Notice that both the pronoun and the verb forms change:

• du klickst → Sie klicken
• du kannst → Sie können

Both are possible, but they are used a little differently.

• auf die Schaltfläche klicken = click on the button
• die Schaltfläche anklicken = click the button

So German has two common patterns:

• auf + accusative + klicken
• something + anklicken

Examples:

• Wenn du auf die blaue Schaltfläche klickst ...
• Wenn du die blaue Schaltfläche anklickst ...

Both are natural. The original sentence uses the first pattern.