Wenn der Browser abstürzt, starte ich den Computer neu.

Because wenn introduces a subordinate clause. In German, subordinate clauses normally put the conjugated verb at the end.

So:

• Der Browser stürzt ab. = main clause
• wenn der Browser abstürzt = subordinate clause

That is a very common pattern:

• wenn ...
• weil ...
• dass ...
• ob ...

all typically push the verb to the end of the clause.

German main clauses usually follow the verb-second rule, often called V2. That means the conjugated verb must come in the second position of the main clause.

Here, the whole wenn-clause comes first:

• Wenn der Browser abstürzt, ...

That entire clause counts as position 1. So the verb of the main clause must come next:

• Wenn der Browser abstürzt, starte ich den Computer neu.

So starte is in position 2, and ich comes after it.

If you put the main clause first, then the order is the more familiar:

• Ich starte den Computer neu, wenn der Browser abstürzt.

Because abstürzen is a separable-prefix verb.

In a normal main clause, the prefix separates:

• Der Browser stürzt ab.

But in an infinitive or in a subordinate clause, it stays together:

• abstürzen
• wenn der Browser abstürzt

So the rule is:

• main clause: prefix often separates
• subordinate clause / infinitive: prefix stays attached

More examples:

• Ich rufe dich an.
• ..., weil ich dich anrufe.

It can mean either, depending on context.

In sentences like this, wenn often means:

• if
• when
• whenever

For example, this sentence can sound like a general repeated situation:

• Whenever the browser crashes, I restart the computer.

It can also sound conditional:

• If the browser crashes, I restart the computer.

If you want to stress a more uncertain if, German often uses falls:

• Falls der Browser abstürzt, starte ich den Computer neu.

So wenn is broader and very common.

Because they are in different cases.

• der Browser is the subject, so it is in the nominative
• den Computer is the direct object, so it is in the accusative

Both Browser and Computer are masculine nouns, so:

• nominative masculine: der
• accusative masculine: den

That is why the articles are different.

You can test it by asking:

• Who crashes? → der Browser
• What do I restart? → den Computer

starte is the 1st person singular present tense of starten.

The present tense forms are:

• ich starte
• du startest
• er/sie/es startet
• wir starten
• ihr startet
• sie/Sie starten

So starte ich means the speaker is saying what I do.

Here neu belongs closely with starten and means again / anew / afresh in the sense of restarting.

German commonly says:

• den Computer neu starten
• Ich starte den Computer neu.

So yes, neu starten functions as a common expression meaning to restart.

In the sentence, neu comes near the end because the verb-related information is often placed later in the clause:

• starte ich den Computer neu

You may also see neustarten written as one word, especially in technical contexts. But neu starten is very common and completely normal.

Yes. In standard German, a subordinate clause is separated from the main clause by a comma.

So this is correct:

• Wenn der Browser abstürzt, starte ich den Computer neu.

The comma is not optional here.

Yes, absolutely.

Both are correct:

• Wenn der Browser abstürzt, starte ich den Computer neu.
• Ich starte den Computer neu, wenn der Browser abstürzt.

The difference is mainly focus:

• Starting with wenn der Browser abstürzt emphasizes the condition first.
• Starting with Ich starte den Computer neu emphasizes the action first.

The grammar changes slightly in the second version because the main clause starts normally:

• Ich starte ... not
• starte ich ...

That is because the main clause no longer has the subordinate clause in first position.