Bitte schalte das Radio aus, wenn das Baby schläft.

Because this is a command addressed to one person informally (du).

The verb is ausschalten = to switch off / turn off.
For the du-imperative, German usually drops du and uses the verb stem:

• du schaltest aus = you switch off
• Schalte ... aus! = switch ... off!

So schalte is the correct imperative form here.

In German, the informal singular imperative normally leaves out the subject pronoun du.

So instead of saying:

• Du schaltest das Radio aus. = You switch off the radio.

a command becomes:

• Schalte das Radio aus! = Switch off the radio!

You can sometimes add du for emphasis, but normally it is omitted in commands.

Because ausschalten is a separable verb.

Its two parts are:

• schalten = switch
• aus- = off/out

In a normal main clause, the verb splits:

• Ich schalte das Radio aus.
• Bitte schalte das Radio aus.

So the conjugated part schalte goes near the beginning, and the prefix aus goes to the end of that clause.

Bitte makes the command sound polite: please.

So:

• Schalte das Radio aus. = Turn off the radio.
• Bitte schalte das Radio aus. = Please turn off the radio.

In German, bitte is very common in requests and polite instructions.

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

So:

• das Baby schläft = the baby is sleeping
• wenn das Baby schläft = when the baby is sleeping

That is why schläft comes last in that clause.

It can mean both, depending on context.

In this sentence, it means when:

• wenn das Baby schläft = when the baby is sleeping

But German wenn can also mean if:

• Wenn du Zeit hast, komm vorbei. = If you have time, come by.

So learners often need to decide from context whether wenn is when or if.

Because the verb is schlafen = to sleep, and in the 3rd person singular it changes its stem vowel:

• ich schlafe
• du schläfst
• er / sie / es schläft

So das Baby schläft means the baby sleeps / is sleeping.

This vowel change from a to ä is something you just have to learn with certain strong verbs.

Because both nouns are neuter in German, so they take the article das in the nominative and accusative singular.

• das Radio
• das Baby

In this sentence:

• das Radio is the direct object of schalte ... aus
• das Baby is the subject of schläft

Even though they both use das, their roles in the sentence are different.

Yes. That is very natural German.

• Schalte das Radio aus = Switch off the radio
• Mach das Radio aus = Turn the radio off

Both are common. ausmachen is often used in everyday speech for turning devices off, while ausschalten can sound a little more exact or technical.

It is informal singular, used when speaking to one person you would call du.

The forms are different depending on whom you are addressing:

• du: Bitte schalte das Radio aus.
• ihr: Bitte schaltet das Radio aus.
• Sie: Bitte schalten Sie das Radio aus.

So this sentence is for talking to one person informally.

It actually could. German allows both orders.

You have:

• Bitte schalte das Radio aus, wenn das Baby schläft.

You could also say:

• Wenn das Baby schläft, schalte bitte das Radio aus.

Both are correct. The version in your sentence starts with the main request first, then adds the condition/time clause. That is often very natural in spoken German.