Ek slaap goed wanneer die kamer stil is.

Because wanneer introduces a subordinate clause. In Afrikaans, the finite verb usually goes to the end of a subordinate clause.

So:

• Main clause: Die kamer is stil.
• Subordinate clause: wanneer die kamer stil is

This is one of the most important word-order patterns in Afrikaans.

It can be understood as either, depending on context.

In this sentence, wanneer most naturally means something like when or whenever:

• I sleep well when the room is quiet
• I sleep well whenever the room is quiet

Afrikaans often leaves that distinction to context, just like English sometimes does.

Die means the.

Unlike languages such as German or Dutch, Afrikaans does not change the definite article for gender. Die is used for:

• masculine nouns
• feminine nouns
• neuter nouns
• singular and plural nouns

So die kamer simply means the room.

In a normal Afrikaans main clause, the finite verb usually comes early in the sentence, typically in the second position. Here, slaap is the verb, so it comes before goed.

So the natural order is:

• Ek slaap goed

That is the normal way to say I sleep well.

Here it means well.

Even though goed is often connected with English good, in this sentence it describes how someone sleeps, so it functions like an adverb in English:

• Ek slaap goed = I sleep well

This is very common in Afrikaans, where the same form can often cover both adjective-like and adverb-like uses.

Stil is an adjective meaning quiet or silent.

In die kamer stil is, it describes the room, so it is being used after is:

• Die kamer is stil = The room is quiet

After is, Afrikaans adjectives normally stay in their basic form, so stil does not change here.

Yes. You can also say:

Wanneer die kamer stil is, slaap ek goed.

That is completely natural. Notice that when the subordinate clause comes first, the main clause still keeps normal Afrikaans verb order, so you get slaap ek, not ek slaap immediately after the comma.

Sometimes yes, but wanneer is clearer here.

As in Afrikaans can often mean if, and in some contexts also when. But wanneer more directly means when and avoids confusion.

So for this sentence, wanneer is a very good choice:

• Ek slaap goed wanneer die kamer stil is.

Yes, you need ek here.

Afrikaans normally requires the subject pronoun in a full sentence:

• Ek slaap goed

You usually cannot just say Slaap goed to mean I sleep well, unless it is an unusual special context or an instruction. In an ordinary statement, the subject must be there.