Wenn zu viele Tabs offen sind, bewegt sich der Cursor auf dem Bildschirm sehr langsam.

Because wenn introduces a subordinate clause, and in German subordinate clauses, the conjugated verb goes to the end.

So:

• Wenn ... sind = If ... are
• zu viele Tabs offen sind is the full subordinate clause

This is a very common German pattern:

• Wenn ich Zeit habe, ...
• Weil er krank ist, ...
• Dass sie kommt, ...

In all of these, the conjugated verb goes to the end of the subordinate clause.

German main clauses usually follow the verb-second rule (V2): the finite verb must come in the second position.

Here, the entire wenn-clause counts as the first position:

• Wenn zu viele Tabs offen sind, = position 1
• bewegt = position 2
• sich der Cursor auf dem Bildschirm sehr langsam = the rest

So German says:

• Wenn zu viele Tabs offen sind, bewegt sich der Cursor ...

If you started with the main clause instead, you would get a more familiar word order:

• Der Cursor bewegt sich auf dem Bildschirm sehr langsam, wenn zu viele Tabs offen sind.

Both are correct.

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

So:

• Wenn zu viele Tabs offen sind, bewegt sich der Cursor ...

This comma is required in standard German.

English is often less strict with commas in short clauses, but German uses them much more consistently with subordinate clauses.

Zu viele means too many, not just many.

• viele Tabs = many tabs
• zu viele Tabs = too many tabs

So zu adds the idea that the number is excessive and causes a problem.

Compare:

• Ich habe viele Bücher. = I have many books.
• Ich habe zu viele Bücher. = I have too many books.

In German, offen is very commonly used to describe something as being open in its current state.

So:

• Die Tür ist offen. = The door is open.
• Das Fenster ist offen. = The window is open.
• Die Tabs sind offen. = The tabs are open.

Geöffnet is the past participle of öffnen and often sounds more like opened or emphasizes the action/result of opening.

For everyday situations, German usually prefers offen when talking about the state of something.

Tab is a common loanword in German, especially in computer and internet language. The plural is often simply Tabs, just like in English.

So:

• der Tab = the tab
• die Tabs = the tabs

German uses many tech words borrowed from English, and this is a normal example.

The verb here is sich bewegen, which means to move.

In this sentence, it is used reflexively:

• Der Cursor bewegt sich. = The cursor moves.

German often uses reflexive verbs where English does not. The sich does not mean the cursor is deliberately moving itself in some dramatic way; it is just part of the normal German expression here.

Compare:

• Ich bewege den Tisch. = I move the table.
  (bewegen = to move something)
• Der Tisch bewegt sich. = The table moves.
  (sich bewegen = to move)

Der Cursor is the subject of the main clause, so it is in the nominative case.

You can identify the subject by asking: What is moving?

Answer: der Cursor

That is why it is nominative:

• der Cursor bewegt sich

If it were a direct object, you would expect den Cursor, but that is not the case here.

Because this phrase describes a location, not a direction of movement toward something.

German two-way prepositions like auf can take:

• dative for location
• accusative for direction/destination

Here:

• auf dem Bildschirm = on the screen
  location → dative

If it meant movement onto the screen as a destination, then accusative could be used:

• auf den Bildschirm

But in this sentence, the cursor is moving on the screen, not moving onto it.

Yes. Bildschirm is masculine:

• der Bildschirm = the screen

After auf in a location sense, you need the dative case:

• masculine dative singular: dem
• so: auf dem Bildschirm

This is a useful pattern to memorize:

• der Tisch → auf dem Tisch
• der Bildschirm → auf dem Bildschirm

German often places adverbs and adverbial expressions later in the clause, especially after the core verb structure and important sentence elements.

Here:

• bewegt sich der Cursor auf dem Bildschirm sehr langsam

Sehr langsam describes how the cursor moves, so it works naturally near the end.

Also, sehr modifies langsam:

• langsam = slowly
• sehr langsam = very slowly

This word order is natural and common in German.

Because the subject of the wenn-clause is Tabs, which is plural.

So the verb must also be plural:

• der Tab ist offen = the tab is open
• die Tabs sind offen = the tabs are open

Even though offen stays the same, the verb changes to match the subject.

Yes, that is grammatically correct.

• Wenn zu viele Tabs offen sind = the most natural everyday phrasing
• Wenn zu viele Tabs geöffnet sind = also correct, slightly more formal or more focused on the state resulting from having been opened

In normal speech, many German speakers would prefer offen here.

Not always. Wenn can mean different things depending on context, especially:

• if
• when
• whenever

In your sentence, if is the natural translation:

• Wenn zu viele Tabs offen sind ... = If too many tabs are open ...

But in other contexts, it can mean when:

• Wenn ich nach Hause komme, esse ich. = When I come home, I eat.

So you should learn wenn as a word whose exact translation depends on context.

Yes, but it changes the emphasis and sounds less neutral.

• ... bewegt sich der Cursor sehr langsam = neutral, standard
• ... sehr langsam bewegt sich der Cursor = stronger emphasis on very slowly
• ... langsam bewegt sich der Cursor = possible, but less natural in this context

German word order is flexible, but the original sentence is the most natural everyday version.