Wenn ich das Thema nicht verstehe, bitte ich meine Cousine, alles nochmals zu erklären.

Here wenn means if in the sense of a repeated or possible situation: If I don’t understand the topic, ...

German wenn can mean if, when, or whenever, depending on context. In this sentence, it introduces a condition.

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

So the pattern is:

Wenn + subject + other elements + finite verb

That is why you get:

Wenn ich das Thema nicht verstehe

not:

Wenn ich verstehe das Thema nicht

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 finite verb must come in second position.

So the structure is:

1. Wenn ich das Thema nicht verstehe
2. bitte
3. ich meine Cousine ...

This is a very common German word-order rule:

• Ich bitte meine Cousine ..., if the sentence starts with ich
• Wenn ich das Thema nicht verstehe, bitte ich ..., if the sentence starts with the subordinate clause

Because bitten takes a direct object in the accusative.

So in:

ich bitte meine Cousine

meine Cousine is the person being asked, and that person is in the accusative case.

Compare:

• Ich sehe meine Cousine.
• Ich bitte meine Cousine.

Both use the accusative.

English speakers often expect something like to my cousin, but German bitten does not work that way here.

Because German often uses an infinitive clause with zu after verbs like bitten, versuchen, hoffen, and others.

Here the pattern is:

jemanden bitten, etwas zu tun

= to ask someone to do something

So:

ich bitte meine Cousine, alles nochmals zu erklären

literally follows the German structure:

I ask my cousin to explain everything again

The verb erklären stays in the infinitive, and zu goes in front of it.

Because alles nochmals zu erklären is an infinitive clause with zu. In sentences like this, German very often separates that part with a comma to make the structure clear.

It helps show that:

• meine Cousine = the person being asked
• alles nochmals zu erklären = what she is being asked to do

So the comma makes the sentence easier to read and understand.

Because after bitten, German normally uses the pattern:

jemanden bitten, etwas zu tun

not:

jemanden bitten, um etwas zu tun

So:

• Ich bitte meine Cousine, alles nochmals zu erklären. ✓

The word um is used in other kinds of infinitive clauses, especially ones expressing purpose:

• Ich lerne viel, um die Prüfung zu bestehen.

But that is a different structure.

Alles means everything.

It is the pronoun form used for everything / all of it.
Alle usually means all people or everyone, depending on context.

So:

• alles = everything
• alle = all / everyone

In this sentence, the speaker wants the cousin to explain everything again, so alles is correct.

Both can often be translated as again, but nochmals strongly suggests once more or another time, especially in a situation like repeating an explanation.

So here:

alles nochmals zu erklären
= to explain everything once more / again

Wieder is also possible in many contexts, but nochmals sounds especially natural when talking about repeating something for clarity.

Close alternatives would be:

• alles nochmals zu erklären
• alles noch mal zu erklären
• alles noch einmal zu erklären

These are all common, with slightly different levels of formality:

• nochmals = a bit more formal/written
• noch mal = very common in speech
• noch einmal = neutral

Because nicht usually goes before the part it is negating, or near the end of the clause if it negates the verb idea more generally.

Here the meaning is:

I do not understand the topic

So nicht appears before the final verb in the subordinate clause:

Wenn ich das Thema nicht verstehe

That placement is normal and natural in German.

If you moved nicht somewhere else, the emphasis could change or the sentence could sound wrong.

Because Thema is a neuter noun in German: das Thema.

So in the sentence:

ich verstehe das Thema nicht

das Thema is the accusative form, and for many neuter nouns the nominative and accusative article are the same:

• nominative: das Thema
• accusative: das Thema

That is why it stays das, not den.