Nachts bellt die Hündin nicht, wenn sie schon schläft.

Here nachts means at night or during the night.

It is not the plural noun Nächte. Instead, nachts is a fixed adverbial form. German has several time expressions like this:

• tags = during the day / by day
• nachts = at night
• morgens = in the morning
• abends = in the evening

So in this sentence, Nachts tells you when the barking does or does not happen.

Both are possible.

German main clauses follow the verb-second rule: the finite verb must be in the second position.

So if you begin with Nachts, then the verb must come next:

• Nachts bellt die Hündin ...

If you begin with the subject, that is also fine:

• Die Hündin bellt nachts ...

The difference is mostly emphasis:

• Nachts bellt die Hündin nicht ... emphasizes the time.
• Die Hündin bellt nachts nicht ... sounds more neutral.

Die Hündin means the female dog or the bitch in the animal sense.

German often distinguishes male and female animals:

• der Hund = dog / male dog
• die Hündin = female dog

A learner may also notice that Hündin is feminine, so it takes:

• the article die
• the pronoun sie

In this sentence, nicht negates the main idea:

• Nachts bellt die Hündin nicht = At night, the female dog does not bark

Then the wenn clause gives the condition:

• wenn sie schon schläft = when/if she is already asleep

So the structure is:

• main clause + nicht
• then subordinate clause

German often places nicht toward the end of the part it negates. Here it negates bellt, so it appears before the subordinate clause.

It can feel like either, depending on context.

Wenn is commonly used for:

• when/whenever in repeated or general situations
• if in conditions

In this sentence, it most naturally means something like:

• when she is already asleep
• or if she is already asleep

Because the sentence describes a general situation, English might translate it as when or whenever.

Because wenn sie schon schläft is a subordinate clause.

In German, subordinate clauses introduced by words like wenn, weil, dass, obwohl usually send the finite verb to the end.

So:

• sie schläft schon = main clause word order
• wenn sie schon schläft = subordinate clause, verb at the end

That is a very important German pattern.

Here schon means already.

So:

• sie schläft schon = she is already sleeping / she is already asleep

It suggests that the dog has already fallen asleep, so barking is no longer happening.

Depending on context, schon can have other uses in German, but here already is the natural meaning.

Here sie refers to die Hündin.

Since Hündin is a feminine noun, the matching pronoun is sie.

In English, you might say she because it is a female dog. In German, that also fits grammatically because the noun itself is feminine.

Because German uses a comma to separate a subordinate clause from the main clause.

So:

• Nachts bellt die Hündin nicht, wenn sie schon schläft.

The part beginning with wenn is a subordinate clause, so the comma is required.

Yes, absolutely.

That version is also correct:

• Wenn sie schon schläft, bellt die Hündin nachts nicht.

When the subordinate clause comes first, the main clause still follows the normal German pattern: the finite verb comes first within the remaining main-clause structure:

• Wenn sie schon schläft, bellt ...

This version puts more focus on the condition when/if she is already asleep.

In many cases, either can work.

The verb schlafen literally means to sleep, so sie schläft is literally she is sleeping. But in English, depending on the situation, she is asleep may sound more natural.

So here:

• wenn sie schon schläft
  can be understood as
• when she is already sleeping
• or more naturally, when she is already asleep

Both express the idea well.