Wenn ich sie streichle, beginnt die Katze sofort zu schnurren.

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

• Main clause: Ich streichle sie.
• Subordinate clause: Wenn ich sie streichle

That is one of the most important German word-order patterns to learn.

Because the sentence begins with the wenn-clause, and that whole clause takes the first position in the sentence. In a German main clause, the conjugated verb must still come in second position.

So the structure is:

• Wenn ich sie streichle, = first position
• beginnt = second position
• die Katze = subject after the verb

This is very normal in German:

• Wenn ich nach Hause komme, esse ich.
• Wenn es regnet, bleiben wir drinnen.

It can mean both, depending on context.

In this sentence, wenn most naturally means when or whenever:

• Whenever I stroke/pet her, the cat immediately starts purring.

If the speaker is talking about a repeated, typical situation, whenever is often the best English idea.

German uses wenn for both conditional and repeated-time meanings, so you have to understand it from context.

Here, sie is the direct object of streichle, so it means her or it.

Why do we know that?

• ich is clearly the subject, because the verb is streichle = I stroke/pet
• so sie must be the thing being stroked/petted

With feminine nouns, sie can look the same in nominative and accusative, so you often identify its role from the verb and sentence structure.

Since die Katze is feminine, sie refers back to it naturally.

Yes. German can do this, and English can too:

• first refer to something with a pronoun because it is already known
• then name it again in the next clause

But many learners may find this version more straightforward:

• Wenn ich die Katze streichle, beginnt sie sofort zu schnurren.

That version is also completely correct and may feel more natural if you are introducing the cat right there for the first time.

The original sentence suggests that sie is already understood from the context.

Because beginnen is followed by zu + infinitive when you say what someone or something begins to do.

So:

• beginnen = to begin
• schnurren = to purr
• beginnt zu schnurren = begins to purr

This is similar to English begins to purr.

Other examples:

• Er beginnt zu lachen.
• Wir beginnen zu arbeiten.

Zu is the marker used with many infinitive constructions in German. Here it works like English to in to purr.

So:

• schnurren = purr
• zu schnurren = to purr

After beginnen, anfangen, versuchen, and many other verbs, German often uses zu + infinitive.

Sofort is an adverb meaning immediately/right away, and in this sentence it sits in the middle part of the clause, before the infinitive phrase zu schnurren.

Structure of the main clause:

• beginnt = conjugated verb
• die Katze = subject
• sofort = adverb
• zu schnurren = infinitive phrase

That placement is very natural in German.

Yes. That would also be very natural.

For example:

• Wenn ich sie streichle, fängt die Katze sofort an zu schnurren.

Both beginnen and anfangen can mean to begin/start.

A rough difference:

• beginnen can sound a bit more neutral or slightly more formal
• anfangen is very common in everyday speech

Both work well here.

Because German separates a subordinate clause from the main clause with a comma.

So:

• Wenn ich sie streichle, beginnt die Katze sofort zu schnurren.

This comma is required in standard German.